Is this radically abnormal? I'm using about 100 mods. Am I just asking for trouble? I

In my game with only about a dozen mods my first save was just over 4MB. My current game with about 50 mods was 5.5MB when I saved at Riverwood. So 6.9MB doesn't sound crazy to me especially if you've got mods that add encounters and NPCs.

Sounds okay to me. Vanilla saves are between 3-4mb, and certain mods add to that quickly. The real thing is to watch is how it grows. If you start at 7mb, that isn't exactly an issue. But if after an hour of play you're at 20mb then you definitely have a problem that needs to be fixed.

With about 100 mods that starting size is somewhat expected. I wouldn't worry too much unless it starts to climb fast.

You could post your load so that we could spot potential issues. Just in case.

Question: I've got around 45 mods. My savegames started from 3 megabytes and now they are around 10. Is this "normal"? I've played arouynd 20 hours.

10MB is still perfectly normal. Especially if it has clocked significant amount of hours.

Yesterday I started a new game with about 50 mods. My first save is 5.47. So I agree with the others here: your save size sounds pretty normal.