i'd agree that the type of insulin can be a factor but would also say that if he can't give you a reason other than you having a big baby last time then i would certainly seek a second opinion.

is this an endo or an ob or a GP? i found that the endo i saw when i had GD was very helpful & really looked at me & my situation as an individual case whereas the obs i saw at the hopsital very much lumped me in a 'GD' category without really taking into account my individual circumstances etc. obviously this isn't the case for everyone (mine was at a public hopsital so not the same ob every time & large patient volumes) but i found the endos to be great.