Those items are big because too powerful. It's a way to avoid player carrying too many of them. I agree land mine should be the same size as grenade (1x1).
The same apply to size difference between saw-off shotgun and grip shotgun. One take 4x1 in order to fit in rider jacket, the other one a huge 5x3. It's a way of balancing the game.

[1.07.153006]
Land Mine - 2x2
Bear Trap - 7x3

@Geez Maybe close this ticket because it's obsolete