Are the vines rooted in multiple places along their stems? If the vines aren't lifted periodically you can get small roots like that. They will try to form tubers where ever they have roots down, so to get big tubers at the base, all other roots need to be discouraged. That's my first guess.

Sweet potatoes are not normally ready until late September. Other than that, they are not demanding of anything but space and loose soil. The major edible is the roots and they take time and heat to fill out.
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Did you plant the whole potato? Also did you plant it in real soil?

Yeah, that was my question, did you plant the whole potato? We started our potato in a pot also, and when the slips got to about 8", we cut them off and planted them in a tilled area of soil. 4 months should have been enough time if you planted them correctly.

I plant sweet potatoes every year for about six years now. I usually dig it out in October. Always had lots of good size tubers. I had one tuber last year which was unusually large. My GF insisted to weigh it and took it to the scale its weight was 3 pounds and 6 ounces. I usually wrap it in foil paper and cook it in the oven We shared together it was sweet and delicious.

I've already been stealing some nice medium sized tubers from my plants, so I didn't think anything of the timing, but sure enough that could be it. I've been stealing mine as an excuse to be sure the voles aren't getting them. Little buggers will move right in and eat every single tuber (I only grow 10 or so plants, the husband doesn't care for sweet potatoes).

Wayne5 and Edweather..Yes I planted the sweet potato in real soil. I don't know what fake soil is. (seriously I don't) When I started it in the very beginning, it was just a wal mart sweet potato that I layed horizontally in a pot and placed Miracle Grow potting soil around it until half the potato was covered. Once the potato had vines 2 inches long I cut those "the slips" with the eye part and transplanted it into our vegi garden where we grow all our other vegetables.

sunnibell7, I did let the plant get really viney...(or a lot of vines on it taking over the ground area of the garden.) I didn't know not to. Like I said this is our first year to try this.

foolishpleasure, wow that is huge!

Sunnibell what are voles? Do you mean Moles? Or Golphers?

So to recap what all of you are saying...Wait, give it more time harvest them in October. And lift the vines so they do not root into the soil. Am I correct here?

Yes, wait longer. Slips should be about 5 inches long or so. Real dirt is soil that is firm...not leaves or just compost which likely would make very long skinny roots.

Planting sweet potato slips in early April would be a problem, due to everything I read says they're a warm weather crop. I usually put my slips in the ground about the same time as my tomato plants, mid-May. Towards the end of August, or early September, they show purplish flowers, and you might see the ground cracking when the potatoes expand. Usually about the third week of September, mine are ready to be dug, and then layed out for a couple weeks of curing.(in the shade)

Voles are little mouse-like creatures, but with short tails. If they aren't ravishing your gardens, you might think they are kind of cute, like little wild hamsters. However, since they eat almost entirely vegetarian, getting them in your garden can be a real hair pulling experince. They make the Huns look like choir boys. Some of them make small tunnels, hard to spot, unlike mole tunnels, and other make well-chewed runs above ground. I had good luck trapping them in winter, but now they are moving back in from the fields, and it's a little more difficult to trap them now. They really like mulch, but also I think the sweet potato vines must be vole heaven- cover from above and giant sweet roots to eat, weeha!

As stated the slips should be longer than 2", but if you got them to grow they should have made potatoes. Probably not in the ground long enough. It seems like forever before they really take off. Mine really seem to do business in the 3rd month. We have a small (4'x6') patch with 4 slips planted in early June. I cut slots in landscape fabric and stuck the slips in. There are some nice potatoes recently formed.