1) We absolutely can ballpark the density of Surtur, I already cited more than one statement that states that he's made of magma and magma has a known density, even using the low end it's over twice the density of water.
Basalt magma: 2.65 to 2.80 gm/cm3 Andesite magma: 2.45 to 2.50 gm/cm3 Rhyolite magma: 2.18 to 2.25 gm/cm3
https://www.quora.com/What-is-the-density-of-magma http://www.usu.edu/geo/shervais/G4500_PDF/45Week1B_Magmas.pdf2) Already cited the statement that show he starts at 800 meters and that it was his minimum size, ergo he was already 800+ meters by the time Hulk punched him, Hulk was merely shown bigger so we could actually see him on the screen instead of him being a dot, if you scale Surtur to the palace 800 meters is about right.
If you want to ignore the official statement and go with 100 meters, then you can never debate using authorial intent or author statements again.
That Hulk only did it with a leaping punch is not relevant, because Thor survived a leaping punch with Hulk' full bodyweight behind it, just like Surtur recieved.