Wouldn’t that make it a bit top heavy?
It will probably make it too heavy in SFS yes.
In real life, the second stage is propelled by liquid hydrogen and oxygen, but liquid hydrogen is so little dense that even a large quantity of it is not that heavy. On the contrary, the booster that runs on solid fuel is way much heavier despite his size.
If you want to make it both realistic and able to fly, a solution is to partially empty the fuel of the second stage to alleviate it.