We all tend to be very front-of-body centric. Most people have tight illiopsoas (hip flexors) and large quads in relation to their glutes and hams. This is a big reason for the rounded back. The rounded back helps engage the quads and keeps the hip flexors shorter.

If you seek balance in your legs, then most need to stretch those hip flexors and lift weights for hams and glutes. I incorporate core movements when I lift for hams and glutes so I also get the core involved. May favorite is with dumbells. I do a backward lunge and return the leg forward without touching the ground. I balance for a second or two and then repeat the lunge. 3 sets of 15 with 50 lbs.