actually with all the bonus's in the game the defense has a higher %. Not the attacker.
An answer worthy of a politician...
The percentage for the defensive bonus may well be higher, but the cumulative effect of the attack+defense bonus far, far outweighs the percentage bonus of defensive GBs. (It starts off hardly noticeable, but the higher the boost level the more pronounced it gets... At 150% it is so imbalanced it's nearly a joke.)
The OFF/DEF imbalance means that a boosted attacker gets a significant DEF boost that limits damage, whilst also getting an ATT boost that causes damage. Meanwhile, from a defensive PoV, to take advantage of the DEF bonus a defender needs to use units with a high DEF stat - otherwise the increase is negligible overal and provides no real protection against a boosted attacker - meaning the best defensive boosts are gained using heavy units who end up with inferior offensive stats compared to the attacking counterparts.
There are excellent posts
here and
here that argue against "Defensive GB is better" being the case, and
another here that shows the "defensive bonus is higher" myth for what it is by showing an "effective ATT/DEF bonus" relative to the solely DEF bonus by comparing two equal units.
Basically it's the case that defenders take damage from the boosted ATT, but deal no damage due to having a basic attack which often fails to even dent a DEF boosted attacker. When a 150% ATT/DEF boosted attacker
can defeat a 550% DEF boosted defender without losing a unit then that should tell you that something's wrong.
