The smaller the device, the smaller the filter, and thus the shorter the effective life of the device.
I've recently been reading about folks who bring gravity-fed filters (Katadyn Base Camp) to the Boundary Waters and they only last about 5-7 days of use - mostly due to the suspended particulates in the water. They're finding that pre-filtering with a coffee filter helps a lot.
I myself when the route of the Miox purifier, which kills the nasties, but leaves them there as a nice protein source :-)