I think sometimes that a tough character can pull off a wimpy animal though. Go-on Gold's mecha was a chicken ...
LOL I'm tempted to quote this and be done with this topic.
Definitely, in series where the mechs are animals, there's a certain bias in "which animal each team member gets". I think it has to do with how the female Sentai members are treated in general. This is what I think is going on in the decision-making; the female members(and in extension, the mechs they use) are effective and powerful in their own way, but never in terms of brute strength/raw power. Their standout traits are rarely strength, but heart, skill, speed or just pure guts.
To date, the only female Sentai member whose standout trait in the team is strength is BoukenYellow, who has basically rock crushing scoopers as her unique weapon, drives a bulldozer, and her roll call being "the strong adventurer". While BoukenPink's standout trait is tactical analysis.
To be perfectly honest now, I don't have any problems with this trend. It does say something to how Japan(or at least the people who make Sentai) view women, but I don't necessarily think it's bad or needs to be changed.