I got my first nice SxS shotgun recently and I’ve noticed that it is difficult to install and remove the forend from the assembled gun. It feels like it’s compressing a spring and has to be pushed on and pulled off with more force than I like. Is it normal, is something wrong? YouTube videos of other shotguns seems like the friend snaps on with little to no effort.

I played with it some more today and it appears like it’s just tight fitting. Everything works as expected.

The gun's hammers should be cocked before you install the forend. You probably already knew this.

Merkel clearances are notoriously tight. I have three Merkel side by sides, still waiting for them to get a bit looser!

I have a Merkel 47 sle. Those guns are built to very close tolerance. I always put the gun up with the springs in the uncocked position. I could use snap caps but I usually use a piece of wood held in front of the firing pins with the receiver separated from the barrels. Then I dry fire the firing pins into the wood block.
I haven't noticed a significant difference in the effort required to assemble & disassemble the gun with the springs cocked or uncocked. Sounds like you got it but as already stated, they are tight. My gun is a side lock. I am not sure if they are different than the box lock guns or not. Enjoy your gun, I hope you like yours as much as I like mine!
PS: One thing I have done in the past was to use a soft piece of wood or anything that won't mar the finish, to gently & carefully pry the forearm from the barrels. I am not a gunsmith but I would not use a lot of force. Since you got it off already, you probably have a pretty good feel for how much leverage it takes.

My 280EL was so tight I couldn't figure out how to get it together upon its first arrival. I was afraid i was going to break the splinter fore end. I called Merkel USA and spoke to someone in customer service. He recommended putting the splinter fore end on with the gun cracked open slightly. That method worked. After putting a few hundred rounds through the gun it is still extremely tight but much better than when brand new.