My son drank beanstalk+breastmilk before 1 and he doesn't drink much either, but after that I changed to 森永and he drinks a lot since then. Actually formulas shouldn't be the main source of nutrients after one year old so you might add the portion of solid food after that! But if you really worries you might consider switching brands and you can try with trial size milk powders first!
Heard that Nestle (nan) can get baby fat but actually I don't think you should worry too much about the size of the baby.
First of all too fat is also not good as it can cause burden to baby's unsophisticate organs such as heart, kidney, etc. Second if baby do not refuse to drink and eat then should have no problem. Some previous suggestion is good too such as giving a little more solid food can let baby receive various source of nutients. It is good to let baby to clean up bowel from time to time, so suggest to give baby to drink Kiwater daily or drink "open milk tea" once or twice in two weeks.