"in for a penny, in for a pound"

In for a penny, in for a pound (idiom):

"If something is worth doing then it is a case of in for a penny, in for a pound, which means that when gambling or taking a chance, you might as well go the whole way and take all the risks, not just some."

Americanized form: "In for a dime, in for a dollar."

It's an idiom for indicating that you might as well commit fully to the venture being referenced.

That's my take on it anyway.

I've also heard the related proverb (with a slightly different twist, though): “he that steals an egg will steal an ox”.

What it actually means is related to crime: if you steal a penny and will go to jail for it, you may as well steal a pound. Either way you're in as in in jail (or gaol, depending on which side of the Atlantic will be prosecuting you). It obviously hails from a time when a penny and a pound were worth much more than they are today.

So if you've already violated a principle to any degree, it doesn't matter how far you go past the violation threshold, so you may as well derive maximum benefit.