BUT that's not what most people want - typically people want muscle definition & you can absolutely get that with HIIT particularly where some weights are incorporated, but they don't have to be heavy or necessarily increase.
Well, I could call this nonsense. But I'm not rude or dismissive of other people's advice in the Exercise section - I've learnt a lot. Not everyone in here trains the way I like to train (I lift heavy & do ballet, and have very lean abs and very strong glutes, as well as extreme flexibility) but it's always interesting to read about people's ideas, experience & advice.
What you're facing @Changedagain876 is a "plateau" - you've seen some of the results she's aiming for, but these seem to have stalled.
This is because - and this is not "nonsense" - your body has adapted to the stress you're putting it under. So YES, you will need to do something different - and adding weight is one of the most effective things you can do.
Personally, I could care less about a "big bum" - I'm just happy that my glutes are strong & hard, and support my movement.
OP you can't spot reduce or spot bulk. It needs to be part of a long slow programme of progressive overloading and progressive training to failure. If you want a "big bum" -you're looking at what is called "hypertrophy" - a scientific way of saying muscle growth (as opposed to "atrophy" or muscle wastage).
Perhaps look up exercise and hypertrophy on YouTube etc, and see what appeals?
I really like MegSquats, and this video about starting out at the gym, is gold.
She's not pushy in terms of her own programme: she gives really good straight forward advice & I always think that I can do a scaled down version of what she does.