I don't know if this has been answered yet, but wasn't Caylee still not potty trained? I thought I remember reading a text where Casey was complaining about how she hoped her daughter would be potty trained by her third birthday. I agree about the shorts-that they probably still fit her, but I don't think she was potty trained.
The potty training thing seemed odd to me at the time. Girls usually potty train quicker than boys. My guess is that Caylee wasn't potty trained because:
1. Casey probably didn't bother, or tried but didn't stick with it.
2. All the turmoil in that poor little girl's life probably prevented her from getting potty trained. I know that some kids will fight potty training as a way to "control" (for lack of a better term) the parents and to keep the parents nearby.
I think a Pull-up diaper was found near Caylee's remains; but I do think that the shorts still probably fit her. They were stretchy.