A subject near and dear to the heart! Yes, it is light indeed that is the prime factor. Most Drosera are as light hungry as cacti. Often, in terraria this is an issue from the glass itself limiting the usable spectrum; another consideration regarding the use of artificial lighting is the age of the light tubes which become less effective over time and need replacing long before they burn out. As for humidity being a cause for larger size, I think not except possibly from etiolation d/t the aforementioned light issues. Nutrition is also important and indoor plants rely on you to provide this. Try blood worm meal, and mist lightly after feeding. The best and largest sundews I grew were put outdoors for the warm months after gradual acclimzation to sunlight. You need to go slowly to avoid sunburn. All in all, the closer you can place the plants to their natural habitat conditions, and the more you can maximize ideal condions as regards nighttime temp. drops, water and substrate purity, seasonal requirements like rest after flowering and etc!, the better and larger the results will be.
Plants also benefit from a hands off position so provided they are doing well, leave them be where they are happy.
addendum: another factor concerns the roots which need room to grow without hinderance from compaction. As grow the roots, so grows the plant. The health of the roots is essential and in general the bigger the pot the better. Annual repotting is also beneficial and sort of acts like restarting your computer.
