- Larger mesh objects use both more space and more Land Impact. If you pay $300/mo for a sim but you scale everything up way too big (which most people do) you're essentially throwing away $225 every month because if you scaled everything down closer to 1=1 you'd be able to fit 4x more content in your sim.
- Size is relative. If you make your avatar larger, everything around you appears to shrink. Make your avatar smaller and everything around you appears larger.
- Animations can work with small size differences, but not large size differences. Take a dozen average adults and you'll get less than a foot difference in height compared to SL where the average adult human avatar is anywhere from 5' to 8' tall. This is why your animations & furniture rarely work well.
|A bunch of furniture at typical SL sizes. My 5'7"(6' in those shoes) avatar for scale.
|The same furniture reduced to 1=1 scale. The prim floor is to show how much space is saved.
Here's a more detailed article about scale in SL if you want to read more.