I got a similar reaction every time I bought a carload of mulch and helped load it and when I turn down help loading the fifty-pound bag of dog food I buy every month. I keep getting comments about how strong I must be. My internal response is that it's really not all that awesome to be able to heave things around for a short time--"strong" is the people who load all this crap I buy onto and off of trucks, etc.
It makes me wonder if it is my appearance (short, fat, elderly) which makes people surprised about my ability to carry things. Or is it that Americans in general have gotten in the habit of having others do things for us? Perhaps it falls somewhere in between, but I feel a little weird being admired for something that really isn't all that special, although I will cherish the comment the Home Depot cashier made on my way out.
"I sure wouldn't want to mess with you!"
*snort*