a few simple facts:
The targets have health equal to: tickets/targets
the bigger the bomb, the more tickets that you can destroy
AND the bigger the bomb, the bigger the damage radius.
A 500kg bomb direct hit will give -14 tickets, but close to it will still reduce tickets. but a 50kg or rocket, has to be a direct hit to decrease tickets, and aa direct hit wil only reduce 0.5 ticket (so even 1 direct hit, will not show up in the ticket count)
EDIT:
look at post #5 in this link to see the amount of tickets a bomb can destroy:
http://forum.yuplay.com/index.php?showtopic=32