Attacking someone

12x your size: You lose 8% attacking force
4x your size: You lose 8%
3x your size: You lose 8%
2x your size: You lose 8%

1x your size: You lose 8.00% attacking force

1/2 your size: You lose 11.31% attacking force
1/3 your size: You lose 13.86
1/4 your size: You lose 16
1/5 your size: You lose 17.89
1/6 your size: You lose 19.6
1/7 your size: You lose 21.17
1/8 your size: You lose 22.63
1/9 your size: You lose 24
1/10 your size: You lose 25.3
1/11 your size: You lose 26.53
1/12 your size: You lose 27.71

Though, 1/4 to 4x is the Humanitarians limit on Primary. Basically, if you're bottomfeeding on Primary (attacking 1/4 your size), expect to lose about double your jets in the attack.

This increased attacking expenses is somewhat counterbalanced by the decrease in construction cost though, so it's not that big a deal.

Also, attacking someone larger in NW doesn't cause you to lose less units than before.