When a flexible suction cup is pressed against glass, its curved body flattens and some air escapes around the rim. After release, the rubber tends to recover its shape, but the sealed rim traps a space whose volume increases and whose pressure falls below that outside. The cup is not actively “sucking” itself onto the glass. The higher atmospheric pressure outside pushes it against the surface. In an idealised model, the maximum holding force is approximately the pressure difference multiplied by the covered area.
A smooth, clean surface matters not because it is more adhesive, but because it allows the rim to form a continuous seal. A rough wall, dust or a small scratch leaves passages through which air can enter. As the pressures equalise, the cup loosens. Lifting one small section of the rim is particularly effective because admitting air locally can quickly destroy the pressure difference across the whole cup.
A larger cup therefore has greater potential holding force, but actual performance also depends on the material’s elasticity, rim shape, surface condition and direction of loading. Pressing firmly establishes low pressure and a seal; sustained support comes from the pressure difference, not from a permanent vacuum or glue-like material.
https://www.physics.purdue.edu/demos/display_page.php?item=2A-01
https://physicslabs.colorado.edu/demos/fluid-mechanics/statics-of-fluids/atmospheric-pressure/suction-cup/
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