A typical Mercury/Mariner installation (for smaller engines at least) uses the Commander engine control set. (If you have a different setup, then the following may not apply). This has a few features that will lead to a stiff control action.
Firstly, the control unit is largely mechanical. Inside the control box is a set of cams. When you take the unit out of neutral, the control cable to the gear will shift and put the engine in forward or reverse. When you push the lever past this, the gear cable stops moving and the throttle cable takes over. If the unit is new, then these cams should be already well greased. Likewise for the mechanical cables linking to the engine.
Secondly, the control box has a friction adjustment. On my own setup, I find this to be fairly crude. Then again, it too is a mechanical adjuster...much like the adjustment on an exercise bike that allows you to make it more difficult to cycle.
Finally, I found that there's also a bit of resistance associated with the linkages at the engine side (gear more so than throttle). The control cables slot into runners, which in turn control gear and throttle.
Somewhere in there is you're problem (if you consdier it a problem). I'd prefer the engine controls to be stiff but that's just from a safety point. If you fall against a loose control unit, things can happen pretty quickly. Check for tight turns in your cable routing as this can have a big influence the stiffness of the controls also.
Hope this helps.