Instagram has clear usage guidelines that govern the collection and use of user data. Downloading or enlarging profile pictures without user permission may violate these guidelines and may result in your account being suspended.
If Instagram allowed profile pictures to be enlarged, users could be notified when someone views or enlarges their profile picture. This could lead to unwanted tracking and possible harassment.
Even if you could enlarge the profile picture, the quality of the image would often be limited. Instagram profile pictures are usually small and not available in high resolution. Enlarging a small image would result in significant quality degradation.