Like already mentioned, it could be a case of capsular contraction. But, I would also say that a lot of it will depend on the surgeon's technique and skill, along with what the patient asks them to do. They can make a tight pocket for the implant to sit in which will make them feel firm, and they can also open it up a bit more to give it space to squish. However, I think generally speaking silicone is going to be softer than saline in most situations. Another factor in the squishiness is the fill/overfill rate when talking about saline.