Attacking someone

1x or larger than your size: You lose 8.00% attacking force
1/2 your size: You lose 10.08% attacking force
1/3 your size: You lose 11.54
1/4 your size: You lose 12.70
1/5 your size: You lose 13.68
1/6 your size: You lose 14.54
1/7 your size: You lose 15.30
1/8 your size: You lose 16.00
1/9 your size: You lose 16.64
1/10 your size: You lose 17.24
1/11 your size: You lose 17.79
1/12 your size: You lose 18.32

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.