The more light- the larger they grow.

What am I doing wrong?

You might need some help but it won’t hurt the plant to pull it from the pot to see if you need to repot or root prune it. You can search the threads for the procedure especially helpful is Al Tapla’s thread on that subject. From the size of the plant looks like the later is probably needed.

That is a healthy looking cutting. I cringe seeing some of the "nodes" and worst "wet sticks" people are buying (along with associated prices) but you got a good one. Why? Several reasons. 1. You already indicated it had nice roots.....and I am sure it did as the leaf still looks very healthy. 2. It has a nice balance of variegation throughout the leaf (and stem)....and not TOO MUCH white. I see people buying nodes with "half moons" and even paying estra......the new growth will usually then grow out of the all green part of the stem (and be plain green) or all white part of the stem (and be albino and eventually die). Yours has a nice balance and will most likely continue to grow out well. 3. A nice big leaf and good section of stem that can photosynthesize giving energy for more growth. Finally you can see where the new stem/growth is coming out.....and it looks like it is already expanding. So. Be patient!! You plant, soil mix etc. looks great. As the days get longer, the temps increase a bit, that new stem is going to come out soon! Looks very good!!! My favorite houseplant and I grow and propagate this regularly. Keep the soil lightly moist at all times.....not soggy, not dry. An open mix like that (with lots of perlite and/or chunky bark) can be watered regularly in Spring and summer without drying out. In summer I put my plants outside and water almost daily. The temps are arm and conditions are temporarily "tropical". In winter (my NJ home is cool and I have horrible, dark windows) I actually keep them pretty dry and they slow down a lot. If I had better windows or a greenhouse I would do differently. Enjoy our plant! Once established, and if they don't have LARGE areas of white, these are actually really easy plants despite all the hocus pocus and high prices surrounding them at the moment. .