If you decrease the squish 'area' there is a lesser volume of gas that is being trapped between the piston & the head to jet out into the combustion chamber to promote swirl. It's the degree of swirl that helps to control detonation. The squish area provides that high speed jet of gas into the combustion chamber to give you the swirl.
The higher the speed of this jet of gas, & the greater the volume of it, the better, within sensible limits.
The speed of the jet is to some degree controled by the gap between the piston & the head, & the volume by the total surface area of the effective squish area.
A15's seem to have had offset bowls in order to provide an increased squish area.

The squish area isn't just the flat part of the combustion chamber that is close to the piston, it's the area where the flat part of the piston & the head come together & if this is just a band around the perimiter of the bowl, then the squish area is just the part of this band thats under the relevant part of the head.
The closer the piston comes to the head, the higher the speed of the ejected gas & the more turbulant the swirl.
Some engines had pistons that protruded above the deck in order to reduce the gap above them at TDC. That's back in the days of fairly thick copper asbestos head gaskets.

Squish area & swirl have nothing to do with compression ratio, but if the squish area is reduced in order to reduce the compression ratio, then swirl characteristics are changed & not necessarily for the better.


Both of these are A15 pistons. The used one on the left is a factory item while the new one is 'replacement' I will need to machine the crown of the new pistons by about half a mill or so & this will leave me with slightly more compression & a wider bowl preimeter. This will hopefully increase the total squish area of this piston up to about the same as the stock piston.