Why are interspecies hybrids so often larger than either parent species
While they do not necessarily constitute the majority, it is true that there is a large number of interspecies hybrids whose members are larger in size than the parent species, and there exist explanations that account for this phenomenon.

What you are describing is an example of positive heterosis, or hybrid vigor.  While the rationale for plants is more complex, it commonly arises between related species in mammals as a sort of heterozygote advantage in which the contrast in genetic contribution during conception spawns some phenotypic benefit.  The exact implementation of this varies from species to species, but this link here is a nice, concise explanation for why ligers might be so large.