A loose fit might be due to manufacturing defects or low-quality materials. Untested, cheap accessories from questionable "pop-up" stores often have these issues. This issue can occur with both silicone and leather smartphone cases
Even though this is frustrating, you may still try to restore the silicone case's size and shape. It's worth a shot since you don't know how the specific material will react to heat treatment. With care, some TPU bumpers can be tightened back up and bent edges corrected.
Keep in mind we're talking about thermoplastic polyurethane (TPU) cases - the main material for modern covers. Other materials may not have the same results. If your leather phone case
doesn't fit, you can't do anything.