That is one theory I also suspect but I have yet to see a study that look at physically strength during conflicts between people. If it is true then among homosexual relationships the person with less physical strength should be detectable when looking at who is the attacker and who is the victim.
It might also be non-linear. A person who has half the strength of their partner might be significant less likely to be the attacker than if a person has just 10% less physical strength. It would be an interesting read.
I think things change a lot when a dynamic is already established, and involves boundaries being pushed and probed, rather than being "crimes of opportunity" so to call them.
Based on the data it seems that the rate of conflict is a human constant. Two women living together will have a lower average absolute strength than two men living together, but from the data the rate of domestic abuse is identical. Similar, the strength difference in a heterosexual relationship is going to be greater than homosexual, and yet again the rate of domestic abuse is again identical to homosexual relationships.
It seems that the only thing that changes with gender is that when there is a conflict and it "eventually" develops into physical attacks, which is a strange concept in itself, the person who will be the attacker (or at least be found guilty in courts) will be significantly more likely to be a man if the relationship is heterosexual.
It might also be non-linear. A person who has half the strength of their partner might be significant less likely to be the attacker than if a person has just 10% less physical strength. It would be an interesting read.