To present what I am talking about, the Top5 non-G20 countries at the last 5 Summer Games:
1st
2nd
3rd
4th
5th
2000
8th, 12
10th, 11
11th, 11
13th, 8
14th, 6
2004
11th, 9
12th, 8
13th, 8
14th, 8
15th, 6
2008
11th, 7
12th, 7
13th, 6
14th, 5
15th, 5
2012
10th, 8
12th, 7
13th, 6
14th, 6
15th, 5
2016
11th, 8
12th, 8
14th, 7
15th, 6
16th, 6
You don't have to fix the medal table to see which are the best "second tier" nations at the Games. And then you can have a third tier for even smaller nations. Sure, some countries have it easier because they can collect more medals in athletics or swimming, but that has been always "part of the game", it's fundamentally part of the Olympics.