There are two sides to that.
1. They are made smaller ie. USA has 75cl, UK has 70 cl. Then there is 35cl, 20cl and of course 5cl.
2. Then the other side is I want a nice size dram ... or two
so a small bottle would be no good as it would empty too fast
I see your point though, but air doesn't affect it that quickly so you can enjoy it for a while.