Sorry this may have been asked be or even may be considered a stupid question, but here goes.
Why is the Sig P229 22cal the same as a full size Sig P226 9MM? It is not compact like the Sig P228 or P229 9MM. What is the reasoning for that or can anyone explain?

I'm certainly confused on this question. P229 9mm, 228 and regular 229 are essentially the same size. There is no compact versus full size in any of these. Yes there are differences like folded versus milled slide and the famous -1 frame sizing, but your question is not understandable in those terms. The .22 version is a std lower with the .22 upper and as such is not anything different in size. So maybe ask your question in a different way.

The Sig P228/P229 in a 9MM or 40cal is smaller that the P226, correct. A more compact version.
However, if you buy a Sig P229 22Lr, it is the same size as a P226 not the P228/P229 that the 9MM/40cal are. Do this make it clearer?

Wow. I never noticed that but a quick image search shows them to look the exact same length.

Just picked up a 22 conversion for my 226/220 this week. I'll check this afternoon and see if it also fits on the 229/228.

The 22 conversion for the 229 is the same length as the 226/220 upper. There is a difference for the 229 being with or without rail, but still the same length.

The 220 caliber conversion SHOULD work in a 226 and vice versa, SHOULD.

I’ve heard that a 229 conversion will lock itself onto a 226, so don’t do it.

Double stack mags for the 229 are 226 mags with a different baseplate.

I am not talking about the conversion, I am talking about a Sig P229 22LR pistol NIB. It is the same size as P226 not a P228/P229.

It is not a compact! Why don't the just call it a P226. The gun is full size not compact like a P228/P229 9mm.

The slide is the same length, however the frame is the same size as any other 228/229.