I too found it not that simple.
Smaller is quicker for sure, but is it worth it.
The budget you touched upon is a factor I think sometimes overlooked.
If you have a budget of £50.00 you can buy say 50 25mm figures.
You could of course buy the same 50 6mm figures for less, but seeing as your budget allows for say 300, why not get 300 and actually allocate all of your budget.
I find I paint my 6mm as I would 25mm.
No undercoat showing and calling it shadows.
No paint spillages, if there is some flesh on the collar it gets re-painted.
Shading and highlighting, ‘painted’ rather than dry brushed, no using the dip.
Little detail picked out like facial hair and belts etc..
All in all it takes me about an hour to paint a normal 6mm figure. (not including basing)
My character models took more as they have much more detail.
My zombie ones took less as I only used three basic colours.
But then I am slow as anything when it comes to painting…