Why does the big factories only produce 1.5 yellow belts? That seems so little!

First, consider how much infrastructure is normally needed for that kind of output. The big furnaces are 50x the electric furnaces or 100x the stone furnaces. The big assembling machines are 32x assembling-machine-3's or 80x assembling-machine-1's. So that is a pretty huge amount of infrastructure to be handing you for free and lots of them spread around the map.

Secondly, consider that these big structures take 5 or 6 module slots and can also fit a huge amount of beacons around them, so you should be able to get them saturating 8 blue-belts eventually.

Finally, a planned feature is to increase the speed of the big structures through technology instead of needing beacons.