Flight Envelope & Speed Tape

[flybywire380]

All,

Would someone be able to explain, physically, why the black & amber flight envelope strips on the speed tape become progressively closer together as you climb please?

I am aware of coffin/Q- corner and low & high speed buffet/stall; but would really appreciate an explanation of the physics with reference to IAS/EAS/TAS/Mach behind why, as you climb, the low-speed stall speed increases and the high-speed stall speed decreases?

Thanks in advance