Some camera applications only offer a limited selection of resolution sizes when called by intent. It varies from device to device, but on the HTC devices I've generally been restricted to the lowest resolution available. (In contrast to the Nexus one camera app that offers 2 larger sizes).
Unfortunately there's not really a way I know of to force these applications to provide the larger image size options.. you might have to write your own simple camera app to get around it (that's what I ended up doing).