Experimenting a bit locally: in Toronto, Walmart gave roughly a one-meg jpg scan, Shopper's Drug Mart 2.4 meg, and Annex photo 0.5 meg - but the first two from 35 mm, the last from 120 6X6! (Not happy with that at all).
Unfortunately, I don't know if there's much to do but compare place to place. I highly doubt that even the chains are completely consistent. Not only that, the staff at this Shopper's (one guy) does it himself and seems to know what he's doing - I haven't asked but suspect I could get him to max it out if I asked nicely. None of these are up to the quality of what I could do myself but for a couple of bucks it's much easier (and I can do upscans when needed, using these as a reference).