Hi Nicki, there can be a number of possibilities, I am no expert but my little botanical knowledge-
1. Too much light, the main function of a leaf is to have a surface to carry out photosynthesis ie. provide food for itself, because your phal might be getting too much light, it isn't producing larger leaves.
2. Something affecting the roots might also affect the foliar growth (but there is a little possibility for this as some phals do look okay even though they have no roots)
3. A variation to your feeding habits might also affect the leaf production.
All in all, if its healthy and producing blooms, it might slowly get back to its original form; just my opinion