It depends on what you mean by expansionist. They were certainly more bellicose than the Arsacids whom they deposed, but at least on their western frontier (which is what I’m most familiar with) they didn’t really seek to expand their control for several centuries. When we look at their wars with the Romans in the third century when they first came to power, there several opportunities (particularly under Shapur) when they could have expanded their territory west towards the Mediterranean but they chose not to. The frontier in Armenia, eastern Anatolia, and the Tigris/Euphrates region moved a bit but not too significantly.