Assets don't take up any space in ROM, since they're just a user-friendly way to paint the screen with nametable + attribute data. The assets and asset groups are tool-only, the structure doesn't carry over to the game itself. Monster groups I'm not as sure.
For NESmaker 4.1.5, banks by default carry these types of data:
00-07 - Overworld screen data
08-0F - Underworld screen data
10 - Background graphics 01
11 - Background graphics 02
12 - Background graphics 03
13 - Monster object graphics 01
14 - Databank1
15 - Monster object graphics 00
16 - Palettes and nametable pointers
17 - Text
18-1A - Empty
1B - Music data and sound engine
1C - Object animation data
1D - Game object, HUD, Start + Win screen graphics
1E - Start + Win screen nametables
1F - Static bank - Main game loop, NMI, and initialization stuff