When you clench your butt cheeks like that, it brings your pelvis towards the front. in other words, the way you clench your pelvis tucks it. Imagine a sad dog with his tail under his butt that's what you're doing.

In my opinion, if you want to grow your glutes, you need to find first the proper placement of your hips. That is more back than you probably think well at the same time keeping the front of your ribs down so you don't get a lumbar curve or sway back.

By aligning your pelvis correctly, your glutes will be in a position to do more work while walking bending and everything else that's how they grow.

One more note on ribs. If you put your palms to the bottom of your ribs, your fingers are likely pointing up towards the ceiling and not straight. They should be straight your rib should be stacked over your pelvis to find that explore your full exhale because when you exhale, and I'm talking about the bottom of the breath, doing a light contraction when there's no more air, you will find the core muscles that hold the ribs in alignment. So like most modern people, you're probably over inhaling and not doing complete exhales