It stands to reason that although a mini u-lock is able to capture less (i.e. unable to secure wheels and frame), that when fit properly such as pictured in the OP, that it would make a leverage attack considerably more difficult than it would be on a similarly fitted full size u-lock, owing to the fact there's no room to insert a long pipe inside the 'u'. So I vote Yes, more secure!

Security, of course, ultimately depends on the quality of the lock, which is to say that not all u-locks are equal.