My font looks too big in InDesign

They may be proportional, but you drew yours bigger. The UPM establishes the size of your drawing area. The UPM in both fonts is 1,000, so when the fonts are in use, that UPM value is what the software uses to determine the output size. Yours was drawn bigger so it will output bigger.

Exactly! But shouldnt there be some extra white space in Century then? Just dont get how they can do that, if you are supposed to draw it to match the metrics as Georg says… If the proportions are the same, but they size is not. And both on 1000 UPM.

So you would recommend just drawing it to the metrics I have defined and once I “am done”. I just change the UPM to for example 800 if I think the font is 20% too big in InDesign compared to other fonts.

Yes. But you would need make the UPM bigger. Because that makes everything else relatively smaller.