I've not heard of this before... any credible source to such a claim?

It's more what you do in the game that causes bloating. An earlier issue, which has been patched out, would leave NPC weapons out in the world instead of removing them with the bodies. So, the save file would grow to account for all those weapons. The save file is going to grow in size as you play the game, that is just the way that it is but things like the weapons being around was not helping things.

Now, the problem with overwriting save slots is this is a 5 to 10MB, or larger, database, the more times it is overwritten, the more of a chance a save could become corrupted. Skyrim is much better about this than previous games were, but it's still a good idea to make more slots rather than less, and delete the excess ones later.