What Blue says in 99.99999% true - there are some things that the game may neglect to include when you package a lot - in particular the clothing meshes and hair meshes of sims that are on the lot, and some custom content may be included but pretty much useless to the end user (modular stairs are a particularly good example here); I think this remains true of Pets EP. Prior to Pets, you had to be very cautious about the Lots you uploaded because the game was down-right obsessive compulsive about including every recolour of every item you used (and may have even deleted from the lot during design) when it packaged the lot for sharing. My all time favourite example is a 2x3 lot with nothing on it but the mailbox, the trash can, and some terrain edits - it was 17 MB uncompressed. There were a bajillion mailbox recolours, a bajillion trash can recolours, 20 terrain paints (including one that was a green grass replacement), 2 versions of CEP, 3 mail box hacks, InSIMenator, InTEENimater, and about 30 other hacks/mods, including the nudity hack, the paperboy, and the mailman.
I assume the creator had played the lot a bit before moving the family out. But even so, it was a crazy thing to see.
So it is fairly important that you use Sims2Pack Clean Installer to delete unneeded recolours and objects from the lots you want to share. You can download and read about it here:
http://sims2pack.modthesims2.com/
