Can the steroid in a L5 s1 injection spread into the psoas muscle which attaches to the last 4 vertebrae causing it to shrink?

Let me explain. If you look at the anatomy of the psoas muscle you see the body is in front l5-s1 disc space which usually injected from the back area. I doubt very much that could happen.
Unlikely. While a steroid injection near l5-s1 has the potential of diffusing and including the psoas muscle which is near…..Especially if there was no image guidance, it is unlikely to cause "shrinking" of the psoas muscle. Steroid is an anti-inflammatory which targets swelling and inflammation. Any contracture to the psoas muscle is likely unrelated.

Related Questions

Can training of soft tissue like ballet cause the L5 s1 vertebrae to be anatomically higher than the iliac crest? Due to hypertrophied lm muscles?

No. That cannot anatomically happen. There could be structural differences that you may have (congenital issues), but what you described cannot happen. Read more...