Wrong mag

How can you tell?

That magazine is too big for mp7, so no way it'll fit into the gun. The detail of the drawing is amazing, so I'm surprised the artist would commit this kind of silly mistake.

looks like an m4 short mag... maybe making that an airsoft "mp7" generic copy thing.
since a lot of the copy thing guns in airsoft use "m4" mags/gearboxes as generic fitting to keep costs down.

Well actually mp7's magazine goes into the gun's grip like a handgun, and one could imagine how big the grip would have become if it is to house a m4 magazine. I'm an airsoft player myself and I have never ever seen any airsoft manufacturers modify the grip of their mp7 just so that it takes a m4-type magazine. Manufacturers tend to have their replica product stay true to the real thing as much as possible so it's unlikely they make such mp7 even if it keeps the production costs down.

My guess is that the artist had a airsoft mp7 as reference but didn't know what the corresponding magazine looks like so he/she just went with a m4 mag.