That's because the Yamaha 9.9 is a depowered 15 and uses a 15 block.

The Yammy 8 and 6 2-strokes use the same block.

The Tohatsu 9.9 2-stroke uses the same block at the 8HP (not the 6 as you stated) but their 4-stroke uses the same block as the 15 (like Yamaha).

Looking at other brands isn't very helpful because (other than Honda) all the leading makers use Tohatsu as the OEM under 8HP.

Generally if you look at the higher HP for a given block, the 4-stroke will be around 20+ pounds heavier than a 2-stroke.