Yes, that's true. The sizes are already quite large and maybe we will at least reduce the living space a bit. Because 120sqm I actually find big enough, but my boyfriend thinks it's too small! But he grew up in a house with a total of four siblings + parents and I'm an only child.
I would agree with your boyfriend in this respect. 120 sqm does sound like a lot, but if you still want to include a utility room and stairs/hallway in the sqm number, it will be quite small.
Realistically speaking, people always build bigger because they want it more spacious. If you are still quite young and other acquaintances your age live in 70 - 80 sqm, 120 sqm is already quite big, but if children come along, it will get tight again.
Therefore, maybe – regardless of the low interest rates now – a consideration in this direction would be sensible. Do I want to live with 2 or 3 children in a cozy, small house where it will eventually get quite cramped?
If your boyfriend already earns well now, he will probably earn more in a few years, and you will also contribute your share as an academic.
I don't know if you will regret later that you chose a budget house in your mid-20s instead of just waiting until you are established in your career and could have simply afforded "more"...