It definately needs more light than where it came from which is why the 2 tallest branches are elongated. Their color is lighter as usual in newer stems so I am assuming they are new.
Based on the number of stems it might need repotting, but no more than 1 inch in between rootball and inside edge of pot.
I would also remove all that loose stuff on top. That might promote mold, gnats etc.

Thanks!! I've removed the mossy stuff from the top! Where I have it sitting, it's getting light from 2 different windows.

Nice plant, hard to see about the light from TWO windows. If that's the case, I'd suggest leaving it in that pot (after removing all the lose stuff from the top) & see how it behaves in 1 month.

It won't harm it to be crowded in the pot, where as too roomy a pot can encourage rot (espcially if it's cold) or overwatered. If the stems stay green & rigid, they're fine, if they should soften at all or get mushy that's overwatering & on its way to rot.

I'm assuming the pot has a drainage hole, that's an absolute must.

Mine is directly in front of a ESE window and is not enough light as the new shoots since Feb are a little elongated. Last summer I put it outside under 4 layers of screen and lattice patio cover and leaves got burned.
I do think you probably need it closer to window but be careful depending on the time of year.
IMO they like more light than the experts say.
If the stems start to fall or lean over without rotting then not enough water. I did that once as I tend to underwater.
I have a small one in an 8 inch pot and water it once a month, the monster was in a 12 but now a 16 as I don't want to repot it ever again! It get 3 quarts every 45 days.
Don't expect any new growth until Jan-Feb. They are known to just push once a year.
I would leave in the pot for a while. But if you really want to repot it do it this fall before it thinks about new growth. That will give it time to adjust for that.

I'd skip the repot altogether 'til next year or if you must do it, I'd suggest do it before Fall. Tho' mine had a growth spurt this year in Jan/Feb for the 1st time, it usually does that in Sept; so I'd beware that too.

If you want it to get very large, and make a statement. Increase the size of the pot. Give it fresh mix and watch if grow. They are easy to propagate. Pot size will depend on how fast you want it to offset, and how many you want, they can grow quiet large. Norma

I think I'm probably going to have to repot it. The container it's in doesn't have a drainage hole. I'd love for it to get BIG! Should I go on up to a large container now? Usually when I repot I only go up 1"-2". Also, will regular potting soil be OK?

Since you said its in a 4 1/2 inch, put it in a 6 inch. Use well draining soil with some perlite in it and little or no peat. I expect the rootball to be compacted some so carefully loosen it a bit. Try to use soil with no fertilizer. Since it has a new home environment, and new soil, new pot etc, if something goes wrong you won't know why with all the changes. If you put in too large of a pot there will be a tendency to overwater and it will rot.

Looks like you may not be quite sure of all the needs of this plant..
Maybe, until you get the hang of growing this one, you might want to do as suggested and don't overpot...You are taking a gamble with root rot from a roomy pot in hopes of getting a bigger plant you are not use to growing.
I would get the watering thing and location under my belt first.
Then maybe, just maybe when you are use to knowing the needs of this plant for lets say, at least a year or so, grow HUGE in a bigger pot, without fear of root rot... Just be patient that is all..:-)

First off, I would like to know how far it is from each one of those windows. If it is in between the windows, but still say 6 ft away from both, you're still not giving it enough light. Light is cut by half each foot a you are from a window. Say you get 1000 candle feet at the glass of a south window, one foot away from the window will only give you 500 cf, 250 cf at two feet, and so on. Do you see the problem? Now most people know film cameras, you could say a full sun plant would be able to use a 200 film, partial shade a 400 film, a partial light an 800 film, and low light a 1200 film. ZZ's need more light than they are usually given, they need a full sun/partial shade exposure. They are slow enough growers without crippling their photosynthesis more by giving them a low light position.

Thanks for that mentha...
I will have to move mine closer to the windows then, since mine is at least 7 feet away....No wonder it is not growing.
I have a question though.
Does this change when the sun in lower in the sky from fall to spring when the sun shines right across my room against the wall in the spot it is now?
Do I move it closer to the window the higher the sun gets?
Now the sun is almost completely out of my windows right over my home....
Thanks!

I can't find my Reader's Digest Success with Houseplants ( IMO the bible of all houseplant books! Too bad it's out of print. ) book, so maybe someone can look that up, but it makes sense that with the sun being lower on the horizon, you'd get more light in, but don't quote me on that. Many variables would fall into play like budding up and dropping leaves on trees. If you do move it closer to the window, make sure you do it gradually, say a foot a week until you're close enough, reverse in fall.

I agree with Mentha it is just common sense about more/less light based on how high the sun is. Check out your flooring next time, it shines further into the room when sun is low. My neighbors roof line controls the direct sun good.

Even though the plant does well in lower light, by placing the ZZ in brighter light it performs even better...mine did. Keep the plant away from any hot direct afternoon sun as the plant can burn...mine did. A good bright filtered afternoon sun would work well. Mine is in ESE window and needs more light...but nothing I can do about it.

Since the container it's in doesn't have a drainage hole, it's pretty much a given that I'm going to have to repot it. I already have potting soil and bought perlite last night. How should I mix this (ratio)?

I've only had my ZZ a week (today) and from reading here, I think I need to move it. It does get light from 2 windows, but it's too far away from both of them.

Probably about a third should be perlite, but it really depends on the soil you have how heavy it is and what it is consists of. By heavy I don't really mean weight but how fine the particles are and is it really dense or larger particles and loose.

Yes, it definitely needs a pot with good drainage. Don't go any larger than a 6" pot. They like to be rootbound and will rot if placed in too large of a pot. They also need to be watered sparingly in low light or they will rot fast too.

It will definitely be happier with more light. And most of mine tend to put out new shoots in the winter, not the summer, so be patient.