When I play my acoustic guitar sitting down for a while, i get red marks around the bicep area of my arm. sometimes slight pain. That is the place where I rest my arm against the top of the guitar. I might infer that im getting marks from my arm because my guitar is a bit wide. It measures 16 and half inches wide from the lower portion of the guitar.

The guitar is a yamaha fg700s. Fantasy sound and solid top for $200, but I dislike the wide body, as the martin dreads measure 15 5/8"

Or is this becaues of improper technique of how i rest my picking arm on the guitar?

i dont have this issue when playing electric
It happens to me sometimes. You can just adjust your arm if it bothers you. I don't think its a huge deal unless you're cutting off circulation.
It's Only Rock and Roll, But I like It
it's just something you have to live with as an acoustic guitar player. When I play sitting down, i tend to wrap my arm around the guitar more from behind than on top in order to avoid the pressure on my forearm. It seems to work for me. Is it wrong to do so? I don't believe so. As long as it's comfortable.
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also they sell armrests that sit on the top and side of your guitar never used one but they could be worth checking out i beleive john pierce makes one same guy who makes the strings
Get an old sock, cut off the end, and you can wear it on your arm. Looks stupid. Stops the hurts.