Shaddatic

Ahh damn! I did search for a duplicate but I was looking for darker, not brighter! Oh well, hopefully I've still added something!

I got a really clear image of the issue, should hopefully help clarify

From what I can gather from the ticket, this is working as intended; as Minecraft doesn't have any window scaling support. So if your screen is 1440p, then windowed fullscreen is also 1440p - 1080p would be windowed at 2/3 the screen size, & 720p would be half the screen size.
There are 3rd party programs that can upscale windows, but Minecraft itself cannot do this in windowed fullscreen - it can however in actual fullscreen!

I've found it can be fixed by adding

"firstperson_lefthand":
{     "rotation": [ 90, -90, 25 ],     "translation": [ 1.13, 3.2, 1.13],     "scale": [ 0.68, 0.68, 0.68 ] }

to models/item/generated.json! I haven't tested everything, but shears are back to normal!

Edit 1 & 2: A better solution might be to { "scale": [ -0.68, 0.68, -0.68 ] } or { "rotation": [ 0, 90, -25 ] } for left hand, as this flips the model around; fixing these issues and also fixes the other inconsistencies in each hand! Only the clock and compass should be excluded, as they'd then move the wrong way

Edit 3: This may be linked to the arrow, as it would make sense, when in the non-dominant hand, for it to be rotated outward to be loaded into the bow. However, this doesn't work for left handed players as its always upright in the right hand. Perhaps this property could be kept for the arrow only in either hand! it would be a pretty cool detail to include!