What brand ?
Are they tight at the knees when sitting on the bike, or just feel stiff ? Leather often does require breaking in, so it may be that simple. Is there a stretch panel on the back of the knee, or is it leather ? It's hard to tell on my phone.
You don't want them baggy, and they look like they are fitting pretty well from what I can see.

 

DXR, it's a local french brand. Really good gear at a great price. Paid 144 euros for this pant rated AA, with CE level 2 knee+shin and hip protectors included.

Yes there is indeed a stretch panel behind the knees (sorry the photo aren't the best quality lol).

They're really tight at the knee, like I need to use force to bend my legs and keep them bent. It feels like the knee protector gets caught up, so I'm basically fighting against a piece of stiff rubber to bend my knee. I'll try tweaking the position. My understanding is that the top of the protector should be under the kneecap when standing, and at the level of the kneecap when riding ?

 

My understanding is that the top of the protector should be under the kneecap when standing, and at the level of the kneecap when riding ?

I would say that is at best true of looser textile gear like motorcycle jeans. I'd expect a more form-fit leather pants like yours (and my full riding leathers) to cup around the knee, at least a bit. As @Florida lime says, they do need breaking in, but it sounds like they may be a bit too restrictive in your case. With all my full leathers (3 sets so far) I was not restricted to the point of actually having trouble bending my knees. I would say take one size larger.

 

Have you tried the pants on without the knee protector? Just for checking fit, not riding without knee protectors.

 

Not yet but that's a good idea, thanks !

 

You'll need an under garment for leather so the leather doesn't stick to your legs.
UnderArmor is good as are the usual brands dedicated for riding.

Anyone that's tried race leathers know that they'll stick like glue.
Once you get an under garment under that leather you're free to move when riding.

 

You'll need an under garment for leather so the leather doesn't stick to your legs.
UnderArmor is good as are the usual brands dedicated for riding.

Good point, I do that too but forgot to mention it. As I am almost sure @zx4rrCH as a Swiss also skis or snowboards, long ski underwear also works well, it is what I use under my leathers.

 

Not at all. It is thin and synthetic and works fine even at 35°C. The important thing is that it transports sweat and acts as a liner between skin and leather. Of course I wouldn't use thick undergarmets.