It's not the size of her udder, or teats.. it's how often she refills it when it gets empty... If he's stealing... he's hungry... most of the time they are too block headed to take to a bottle.... untill they get "real" hungry...
Up to you, the calf will prob live on mom, and by stealing from other cows.. it likely won't thrive though...
i'd ship the cow before her next calving though...
you cow/ calf might just be like ours.. Some days, *(years) i think our cows just raise the calves as a community affair.. I also don't know that some cows know which calf is their's, and most don't care who's sucking on them... We have one cow that just seaks out the wimping, skinniest calf on the place, and takes over as it's mother.. She's raised twins every year doing this, even though one isn't hers, and still sucks it's mother..