gap-filling glues like smooth on or epoxies require little pressure, just enough to ensure gaps are filled. If clamped too hard they will ooz out and the joint won't be as strong. Any glue that shrinks like titebond, wood glue, or hide glue you need an almost perfect fit, and lots of pressure is good. I may be mistaken but I think the gap filling ones don't soak into the material like the non-gap filling because they are usually designed to glue nonporous materials together like metal, plastic, and glass. So if there's too much pressure there isn't any glue to hold the material together unlike the wood or hide glue which soaks into the pores so it doesn't matter if a lot is squeezed out.