So you are maximizing the metro length at 2 SG3's? At the Rotterdam metro network that's normal, but on the Simvliet network 3xSG3 is a more normal limit.
On the Rijndam network, M5 was build as a outer-region local line; no high passenger numbers expected, so no need for extra long platforms. Other then that, the platforms are build for 3xSG3.