Godville is 24 MB, a zero player game where you create an adventurer who goes out and quests, collects loot and equipment, rests at town to sell things, tames a pet, joins a guild, etc. you can influence what they do, but they do it all automatically and you can just observe!
Impossible Dungeon is 35 MB, idle non turn based dungeon crawler with incremental upgrades, I really like it!
Not sure what you consider low MB, I couldn't find any more that I'd heavily recommend at 150 MB so I cut it off there