Same thing could be flipped to ask why ladies like larger or taller men... Basic nature concept is that females want to feel protected and males want to feel like a protector.
Now when you find a woman who also likes to be a protector you have a keeper as she is strong and can be dominant which is a fun change from the norm.
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For me though... I have always loved brunette or red heads and someone smaller. Ive dated taller and larger but its just something about a smaller woman to me who has a firecracker personality