Of course a tire gets physically smaller when its tread wears away. But for racing tires that's a small amount - typically measured in 32nds of an inch. Certainly not the roughly 3/4 inch that the OP has told me in PM that his tires are shrinking. While kyle18fan is technically correct about tires getting smaller due to wear, late model racing tires do not have the big knobs that dirt motorcycle tires have. If the OP's tires were wearing about 3/4 of and inch then they'd be worn through the cords and blown out (long before they reached 3/4 inch).
On the other hand, Spotter22 is confusing stagger change with tire wear, then goes off on a tangent about stagger changing due to tire growth while racing. While Spotter22 is correct that stagger can (and often does) change that way, this is not what the OP is telling me he is talking about (in PM). The OP is aware of tire growth and its effects on stagger while racing. That's why he is mystified about how his left side tires are shrinking instead of growing.
I am trying to work with the OP on how his tires are measuring so much smaller after a race. Maybe it is a measuring problem. If not I haven't thought of any other reasonable explanation except for leaks. In my racing past I have had tires leak (mainly due to valve stem problems) that didn't show up while the tires were sitting in the pits - these tires had to build up more pressure (due to racing) to get them to leak, and then they leaked enough to show lower pressure than I originally set them to. So far the OP has measured "normal" pressures (in PM, and I'm trying to get him to define his "normal" for me). So I believe we also have a terminology disconnect too.
By the way, even though it's common practice to measure stagger by measuring tire circumference, the long accepted engineering defined way of measuring stagger is actually via tire diameter. Stagger is defined as the difference in heights of two tires; height = diameter. But since diameter and circumference are related by a constant (pi) you can get away with measuring circumference. I too like measuring circumference since it is easier to do and thus can be more accurate. But be careful when comparing stagger with somebody to make sure they are calculating it the same way that you do. I used to write my stagger notes with my numbers corrected to diameter (dividing my circumference measurements by pi) and when asked I'd divulge those numbers - I got cussed at by some people who just used circumference numbers, and who accused me of divulging "wrong" numbers when actually I was giving them engineering numbers that were about a third of what the circumference numbers alone would yield. LOL