does fitting oversize pistons to an engine raise its compression ratio?

Not sure exactly you're intent here, but l'll try and give you a basic run down.

Compression ratio is just the volume contained in a cylinder at BDC divided by the volume at TDC.

Or a 10:1 compression ratio means that the air (and possibly fuel) get compressed to 1/10th the size... so if it was 500ml its now 50ml.

If you increase the diameter this will just mean that you start with a larger number and end with a larger number, but essentially the ratio will stay the same..

However, this would require that the exact same cavity in the piston existed. So if you got flat tops, or different cavity pistons then this can, and in most cases does, increase compression ratio..

An increase in compression ratio is good as it not only provides greater pressure for combustion, it also increases efficiency of the engine..

The reason it is not increased dramatically is dependant on other factors such as fuel, timing, detonation etc.

So it comes down to the type of oversize pistons that you've used.

Hope this helps

Technically yes...but not by much.
You have a fixed combustion chamber volume and take the gasket volume as constant. If you increase the swept area of the cylinder then technically you increase the comp ratio. However piston manufacturers know this so they increase the dish size on most pistons to compensate for this on standard engines.
So if your going bigger just to increase comp it probably won't happen. Better to select a better piston for the job with a different crown setup to increase it.

so I guess they went to concave pistons in the 14/15 from the flattops of the A10/A12 to compensate for the lowering grade of fuel, and reducing compression without a major rethink of the head?