It probably did - she doesn't mention using any other spells on it.
When she tells Harry how she enchanted the bag to carry everything they needed, she only mentioned casting the Undetectable Extension Charm. She doesn't mention any other spells to make it easier to carry, and it's unlike her to partially describe her methods.
“Undetectable Extension Charm,’ said Hermione. ‘Tricky, but I think I’ve done it OK; anyway, I managed to fit everything we need in here.’ She gave the fragile-looking bag a little shake and it echoed like a cargo hold as a number of heavy objects rolled around inside it. ‘Oh, damn, that’ll be the books,’ she said, peering into it, ‘and I had them all stacked by subject … oh well … Harry, you’d better take the Invisibility Cloak. Ron, hurry up and change …”
- Harry Potter and the Deathly Hallows, Chapter 9 (A Place to Hide)
It is possible that she used another spell on her bag to make its content lighter, but it seems more likely that she fully described what she did, and it was a effect of the Undetectable Extension Charm.