Hi
User is always loaded
If user has a field that is another object, then this another object is loaded as well.
Amongst other costs, Bubble bills based on how much data is being loaded from the DB. So yes the heavier your User object is the costier it gets to operate the most basic operations.
Note: we are talking about fractions of fractions of WU, so that might not be a big deal. Maybe it is, depending on your application and how many other optimisations you’ve done.