it's hard to tell from the picture, but have you seen the pictures Paul B. has posted of his mandy carrying eggs?
assuming yours is a female, she could be gravid? there is a concern she could become egg bound, but often they are able to release these on their own during spawning.
i have heard anecdotal reports of people using epsom salt baths to some effect to help release egg bound dragonets, however before trying anything like this, please verify the diagnosis and the treatment explicitly.
is she acting normal?
how long has she been this large?
how long did it take for her to get this large? was it just over night or has this been a slow progression?
is she the only mandy in the tank?