Preemie growth

I know all babies are different, especially preemies. My son was born at 33w 2d and is 4 weeks actual today. I can tell he is definitely growing; getting chunkier and more defined features, but does anyone else with preemies have any insight on about how fast they grow? I hear the grow fast and I know they’re expected to be behind in their developmental milestones, but I will feel so much less stress once he is bigger and more alert. I did feed him a mix of my breast milk and formula for the first three weeks of his life, but I had trouble producing and I needed to go back on medication that I couldn’t take while breast feeding. Now he is on formula only. Any advice?