Belly Button Scars After Tummy Tuck. Is there a reason for surgeons to make the opening around the navel so large?

To some degree the tummy tuck belly button scar cannot always be predicted. A circular, horizontal or vertical incision can be made but the wound may heal with varying resutls according to tension and the individual's unique wound healing tendencies.

Understandably, the appearance of the umbilicus is of great importance to many patients undergoing tummy tuck surgery; after all, it may be the only telltale sign of having undergone the tummy tuck surgery when the patient is wearing a swimming suit or undergarments. The umbilicus should be positioned carefully; if it is too high or too low, it may be a sign of tummy tuck surgery. Ifit is too round or slit like it may also look "surgical". Most patients prefer a relatively small and oval-shaped umbilicus. I hope this helps.

You have asked a great question!

Honestly, I don't know why some belly button shapes are unusual as the technique exists to make the belly button look as natural as possible.

That is why it is important to look at before and after photos to make sure you like the work of a surgeon.  A  plastic surgeon may be technically proficient but lack the artistic ability to view things in an aesthetic fashion.  For the most part, this is God given.