If you tense your tongue it will narrow, only a relaxed tongue is wide. Just try it in front of a mirror, push the tip against your palate and see if it's as wide as when your tongue is simply relaxing. It's very narrow and pointy when pushing forward. So mewing with pressure may move the maxilla forward, but it will also narrow the front palate which will affect the rest of the face.

Here are some pictures to demonstrate what I mean :

This is only pushing against the palate, notice how much narrower the tongue is than the front palate.

This is pushing on the front palate and sucking on the back palate at the same time, while a big improvement, it still doesn't seem like there's much sideways force on the front palate. Also it's not known what kind of effects forceful sucking has on the face.

Here is just relaxing the tongue against the teeth, somehow the tongue raises itself because of the pressure and then natural suction occurs. Notice how the tongue pushes against the upper palate teeth sideways. One thing I'd like to add is that when opening my mouth and doing relaxed mewing the tongue moves downward because there's no force used either forward or upwasrd, there's no way for me to raise it more than that without using force or simply closing my mouth (in which case a photo is impossible).

Just imagine the tongue slightly higher when the mouth is closed.

This is why I think relaxed mewing will lead to best results in the long-term.