It's hard to say, since you didn't post the program. The problem may exist because you have very large single rungs, since they are inefficient to compile (since a single rung is almost always placed entirely within the same page of the PIC's program memory; so large rungs pack poorly). In that case, you might be able to fix the problem by splitting large single rungs into multiple smaller rungs.

Or the program may just be too big. In that case, use a part with more program memory, perhaps one of the big AVRs.