No. Miniature bulldog just means that genetically, these bullies have been smaller than usual. It is not a separate breed by any means, or any kind of guarantee.
In example, Matilda was supposedly a miniature bulldog. I didn't really want a miniature, I just really liked this particular breeder. Her mother weighed only 32 lbs. Matilda is not even 2 years old and is 52 lbs, and much taller than other bullies we know! She is actually a pretty big female (and in perfect shape according to the vet), so miniature really doesn't mean anything other than "the previous generation was small, so this one probably will be".