: It's been some time since I experimented, but it may be closer to 1.5x.
: That's with no changes to FOV or camera height, though.
Super-specifically, it gets weird due to the fact that Doom uses nonsquare pixels. It's 320x200 but outputs in 4:3 regardless, so the heights and widths in doom are different.
For horizontal resolution (and for how the engine actually thinks about pixels), though, it is 100% a factor of 2. Doom's flats are 64x64 pixels, with 1px mapping to one map unit. Marathon's textures are all 128x128, so on the horizontal scale, they're 2x doom's textures (i.e. they render 2 pixels per doom map unit).
Rendering wise, doom still does 1 pixel per vertical map unit as well, so rendering wise, marathon's textures will cover 128 map units of height, but due to doom's non-square pixels, this is an odd height that looks like 128*1.2 horizontal pixels. Without a doom port that does aspect ratio correction, this isn't present, and honestly it's pretty hard to notice either way. However, it's still mapping 1 pixel to 1 map unit of height, so a marathon texture without scaling, being at 128x128, maps to 128 horizontal and vertical map units.
Visually, this makes textures look roughly 2x as big as they should be when rendered in doom. Like for real, a properly aligned terminal is about as big as one of doom's half-height doors lol. In UDMF you can scale the textures, though, and putting them at 2x scale, you get something that looks visually correct.
I don't know how to do screenshots on this forum, but I do have marathon 2 and inf's textures ripped into a doom wad and can show how this all looks.
