I'm just wondering, at what sort of file sizes should a person start getting worried? With nearly 80 active mods, is a 6.9-7.5mb file size appropriate after about thirty hours of play? As a concept this is new to me, I never modded FNV heavily enough to run into anything like it, so I'm not sure if I'm being justifiably cautious or just plain paranoid

7.5 MB seems fine with that amount of mods active. Especially since scripts are stored in the save games as well with Skyrim. So, don't worry.