You need to include stem length and handle bar geometry in your consideration to get to the so called rider compartment, the actual length from seat to grips. You could adjust that by changing to a different length stem for example. Just to mention that frame geometry alone is not enough to figure out bike fit.ATM I ride Cube AIM XL size and it seems just a little bit too large. Geometry difference between Cube and Nirvana is noticable
The Nirvana frame in L is considerably longer than the Cube in XL but the stem will be shorter. So 'rider compartment' won't be that different. Even if it is, that can easily be adjusted. Probably even without changing parts.Will the L size feel to small for me and be a problem?
Some people might want to have an even longer bike, the Nirvana has a relatively upright and therefore relaxed rider position, but I guess that's not you.
Long story short: I think L is fine for you.