A partner that cannot fight is a partner that will not be able to protect the fry adequately. In many animal contests. The larger bodied animal packs more punch in its bites, may have thicker protective parts, more energy reserves, larger jaws, etc. All else equal, a larger animal will usually win a fight. But there are other factors to consider, and these are particularly important and interesting in cichlids.
Motivation is extremely important in cichlid fights.
Just watch a pair of breeding cichlids defend their nest or fry against much larger attackers. Because of this, parental cichlids are seldom evicted from their broods, and I have witnessed parents effectively turn away predators ten times their body size. A difference of only a few percentage points in weight is enough to predict winners of a staged fight. Body size isn't everything. Francis Neat and colleagues at the University of Glasgow examined fighting behavior in Tilapia zilli Neat et al.