Unless you get a written load calculation you will never know the truth.
Oversizing was and is common. I had 110000 btu, now I have 47000, on low
fire and on the coldest days I only run 12-14 hours. So I am still
oversized. If your unit is 80% efficient and you up efficiency to a 94%,
you can downsize 18% and get the same heat output. But nobody here kmows
how he is comming to that conclusion. Get several estimates, there is
alot to choose from.