They seem hard to level because you try to level them while they're still higher level than the rest. I don't believe that they are actually harder than a beorc at the same level (but I might be wrong)
Edit:
Yeah, if you go by BEXP, it takes pretty much the same.
A lv 30 Laguz to lv 31 takes 30*1.5 = 45*50 + 50 = 2300 exp
For a lv 20/20/10 beorc to 20/20/11 beorc, it takes 50*50 + 50 = 2550 exp.
I'm not sure if it's that way all the way, but it seems that it's close all along.