Do not be alarmed. My Gucci was a purebred, registered female Maltese and as an adult she weighed between 11 and 12 pounds and was 10 inches at the shoulder.
I saw her mother, father and her two other siblings - all Maltese, however, her mother was larger, the size of my Gucci and her brother and sister were smaller like the father.
At 12 pounds you could not easily feel her ribs without pressing a little bit but at 11 pounds the ribs were there and she was at her perfect healthy weight.
Not all Maltese are "tea-cup" size - thankfully, and some. like my Gucci, can be on the top end of the scale size and still be within the breed standard.
The important thing is to not let your Maltese become overweight - if you can't feel the ribs - without pressing down on the dogs side - cut back the treats and go on a regular 2 meal a day feeding schedule - none of the feed- at - will nonsense so many dog owners do.