It's important to measure your horse before purchasing rugs. Ill-fitting blankets can cause discomfort and/or injuries. To measure a horse for a blanket, you measure from the center of their chest to the curve of their buttock.
As for your question, I think that you should make sure that the rugs that fit your larger horse are of the proper size and fit. If they are, then it's fine to keep the rug for your horse. You should buy the smaller horse a properly fitting rug, as well.
I'd recommend doing more research to find out what kind of blanket you need and how the layering systems work.
If you have any other questions, don't hesitate to ask! :)
The hills were bathed in moonlight, the shadows not so stark;
Silver light reflected off his brown hide as he held me in the dark
– I love you, Mudpie!