I love my MPS, it is smaller than a full sized vest, but bigger than a chest rig, and has MOLLE real estate. Carries 2 liters of water, my pistol and 3 pistol mags, a flashlight, radio, speedloader, dummy grenade, dead rag, 5000 bb's, googles (when not on the field) gloves, and tools for basic repairs to the hop up, etc.
In order to carry all this, I bought 3 additional pouches. A double 40mm grenade pouch, a pistol holster with mag holder, and a hydration carrier/backpack.