Every texture, mesh, static mesh, animation, sound, - eat memory on CLIENT. Pickup is a little object that isnt wasting much memory. And even with this giant using of memory, modded servers can be normally played. Standart server - even more so.
Because the server covers most of the major tasks for the client. That is why single player takes up so much damn resources and why it usually freezes for me for a second each wave. because my computer is doing the tasks not a server.
I can actually set the amount of zeds on my server to max and not lag as much as i do in single player because the client is not having to rely on the hosting resources for the game. (I don't do it because thats overkill)