What happens when I capture a petty soul but have only grand soul gems?

When using soul trap on creatures where the soul size doesn't match up with the soul gems I have in my inventory, what exactly happens then? If I capture a soul that is too small for my soul gem, do I automatically waste a large soul gem on a small soul?

So, do I have to be careful which souls I trap in order not to waste any soul gems? Or can I happily trap everything and the souls will only go into the correctly-sized soul gems?

Solution 1:

A captured soul will be put in the smallest available soul gem that will hold it. So if you capture a lesser soul, and you only have a grand soul gem available, it will be automatically put in the grand soul gem. If you have a empty grand and lesser soul gem, a lesser soul will always be put in the lesser soul gem.

If by chance you accidentally fill a larger soul gem with a lesser soul, you can drop it to the ground and pick it back up. Dropping soul gems to the ground causes them to lose the stored soul and this prevents the waste of larger gems filled with smaller souls.