basic biology here.
By squishing the sandwich you are reducing the surface area of the bread. You will still generally chew the sandwich about the same amount per bite squished or unsquished meaning if your bread is squished you will get less tastebud to bread action leaving things that cant really be squished (like condiments/meet) standing out more.
Think about it, flatten out a piece of bread, put mustard on it. go to eat it and it will seem like less bread therefor the mustard will stand out.
Leave it fluffy, put the mustard on it and it will seem like 2 to 3 times more bread.
This is probably off somewhat though cause I haven't exactly studdied how much you chew things of different textures, but I assume you don't chew your bread twice as long if it is squished. This of course only works in situations where the bread is significantly fluffy like rolls and such, cause a square of bread has pretty much the same surface area if you squish it.
Ok, I'm done.