Originally Posted by
gthomson
but most road bikes I see have much smaller freewheel configuration
If you're mostly looking at very old racing bikes, yeah.
Nearly all modern road bikes come with at least 28 teeth on the big cog, with 30 or 32 also being common. Current-generation mid-cage Shimano road derailleurs officially support up to 34T, and they can usually handle somewhat bigger if you force them to.
Originally Posted by
gthomson
why the big cogs on the freewheel are so big? What would be the purpose for this?
Getting lower gearing.
Originally Posted by
gthomson
It almost looks as big as the smaller chain ring.
The cog in your photo is 32T, and the chainring is 42T. It's nowhere near as big as the smaller chainring.
Even if it was, there's nothing esoteric about that. What's needed varies a lot depending on the rider and the rides being ridden. On my gravel bike, the small ring is considerably
smaller than the biggest cog (24-32).