The one on the right is bigger and you can see faint white growth lines . The one on left is not showing growth lines currently. Are you keeping them together? This is not recommended as one can bully the other. Ensure your read the care sheet and provide good heat, light, water, diet, calcium for shell growth and correct environment,
I recommend you keep weight log using kitchen digital scales, weight weekly and this can show growth as well as weight loss so you can respond accordingly.