2 part answer....
A spirit generally doesn't have anatomy.... what they look like is a function of several factors (conjuror's subconscious, spirit type etc). Hence why a fire spirit can look like a column of fire, or a firetruck, or an 8year old girl.... Or for your example a breast spirit could look like a wolf, or a bear, or vorpal bunny...
But, since they don't have an anatomical structure, they can't really eat anything. Agressively 'chew up' sure. But eat, digest, and poop out? Nope.
Realistic form is a bit different. For all intent, they are exactly like the form they copy. They have blood, a pulse, functional organs, and even measurable biosigns! So yes, given enough time a realistic form spirit could eat someone....
But remember, that realistic thing goes both ways. You ain't cramming 150lbs of flesh into your vorpal bunny realistic form spirit in a single night!