What did the roots look like when you planted? Were the plants rootbound in the cells? If so, did you trim them, tease them out when you planted?
I would cut off the flower stalks and check the roots for two possibilities.
1- roots are soggy, rotting, brown or black.
Too much water, poorly draining soil.
2- roots are whitish but growing in spiral same as in the cell.
Need to trim off bottom of roots, tease out roots to promote growth.

The Salvia you refer to is called "Red Hot Sally" and is part of the "Salsa" family of popular Garden Center annual Salvias.

They prefer a well drained soil mixture, warm soil, and a minimum half day sun.
Hope this helps.

I checked the pics of that red hot salvia. It's one of the ones I like. In my area, temps are up at 80-95 F, and they do get much bigger. Perhaps it is the temperature that is keeping them smaller. In other words, they are temps that are OK to grow in but not thrive in, for that particular plant.

One one website, it does mention that cutting off the blooms will have a better result for the leaf/branch growth. Since you mentioned that you left the flowers on until they pretty much fell off on their own, then it's possible the plant spent some time with transplant stress/root regrowth, then supporting the flowers.

I would say that if you remove the spent stalks now, and then wait another 1 month, see how much different it looks at that time.

The main thing right now is that it still looks healthy and is not getting discolored leaves or wilty looks. If that is the case, then it's conceivable it stayed small to support its initial flower flush, and will soon grow out more.