That all sounds like very positive news to me!

If it was just a borrow pit, then they probably didn't create any "extra" problems for you.

8-10 acres of watershed is probably a little too much for a 1-acre pond. (Which is a good problem to have!) Therefore, you should be able to support your pond expansion plan (if all of the OTHER factors look favorable). You might even consider the option during the planning phase to have the ability to divert water around your pond. A big rain on freshly tilled ground is going to try to wash sediment into your pond.

If you are deepening the pond and it is already down to a clay subsoil, then try to doing it during your driest time. Wet clay is no fun to work on.

If it used to hold water better, but there are trees and stumps on the "dam" area, then they are probably the culprits. The path created by a big root after it rots away can leak out a lot of water.

Good luck on your new pond project!