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Is it because much more of the embryonic growth occurs outside the mothers body? This means that many more offspring can be borne to one mother at once, or on successive days, as there is very little development inside the parent. Non egg layers tend to have fewer babies. Not very well put, but do you see my point?
Only in the mammals (and some of the sharks) has the foetus being retained within the mother's body and nurtured there until birth.
Hmm…are you sure about the live birth for tuna? I did think they spawn. But they never cease to amaze...