Thank you for your question. The criteria to understand this statement is not the amount of Christians or Muslims as that would mean that another shared prophet, such as Ibrahim (as) would be the most beloved as Jews, Christians and Muslims all love prophet Ibrahim (as) for example. Rather, the Prophet (saw) being the most beloved creation can be understood in a couple of ways:
1) He (saw) is the most beloved creature to God, and since God has the greatest capacity to love, the Prophet (saw) is the most beloved creature of all time.
2) The pre-eternal reality of the Prophet (saw) is the source of existence of all that is good and so the whole of creation from its beginning to its end loves the Prophet (saw) including Jesus (as), whose very goodness and belovedness is derived from the belovedness of the Prophet (saw).
May you always be successful.