Is it possible to reduce the size of the house after the financing is completed?

Well, that was also a huge unit, 6x9 with an equipment room across the entire width and from the carpenter including assembly
I am planning a double carport 5x6, available from Prikker made of BSH including glulam arches and concrete tiles for just under 5000€. Of course, the foundations and paving are additional, but that is far from that amount. I would never spend 20,000€ on a carport; you can easily get prefabricated garages for that price

A prefabricated garage 6x9m with electric gate, lights, and an occasional socket all-in costs rather >35K€. Metal is possibly a few thousand cheaper than concrete.

So I somehow had different offers. Concrete garage (single) without foundation about €7,500 including delivery. For a double garage, I would rather estimate around €20,000. But in the end, I decided on the carport because with it, the ratio of internal to external length is more favorable and the boundary length to the neighbor is fully utilized. Also, I can simply drive the trailer through the carport, whereas a second gate would be necessary for the garage. But that's actually not what this thread is about. ;)

So somehow I had different offers. Concrete garage (single) without foundation about €7,500 including delivery. For a double garage, I would rather estimate around €20,000.

So what now?
6x9m is not a double garage, that's three of your mentioned elements. On top of that come foundations (€3-5k), the car crane (€2-3k added to the "normal" delivery, if no crane is available), upgrades like the larger (electric) door, electrification inside, drainage, possibly a parapet flashing (€3-4k at this size) so that the garage doesn't look filthy after two years, and so on.

If you compare that to a wooden kit, where you even consciously omit, for example, the floor (paving), the labor, the foundations, etc. - well, then you can only say: That's not how you compare, that's how you make the calculation look good.

We want to reduce the house by 1 meter once again, which will save us another €60,000.

How many floors (above and below ground) does the building have to supposedly save €60,000? Honestly, I can’t even remotely believe that.

That has already been answered, there are three floors. What is there not to believe? Just reverse the well-known rules of thumb...

€60,000 / €2,500/sqm = 24sqm 24sqm / 3 floors = 8sqm per floor

8sqm saved area / 1m shortened building side = 8m is the length of one building side, the other is now x-1

What is so implausible about that? Even with only two floors, it would not be implausible, then one building side would simply be 12m long - also not unusual.

What is implausible about it is that it neither results in fewer windows nor fewer technical installations, so the rule of thumb only works totally but not partially. We had the opposite idea (extension), and the additional costs were correspondingly almost negligible – definitely not five figures, and certainly not starting with a six.