With mass information you could infer much more from pictures. With some sort of standard cube in the picture as well as taking a picture at an angle that emphasizes all three dimensions you could also better estimate the relative volume.

It’s tractable I think, but not from a pic alone.

Yes one could potentially increase accuracy greatly. One big problem would be occlusion.

There is already a solution to this that would be very hard to beat (and one can choose to use or not use an LLM to assist): prepare food yourself and use the information provided by the manufacturer.

maybe, but not always. I could make two identical-looking sandwiches with very different calorie content by changing the type and quantity of sauce on the inside of the bread. I could give you two "pasta with creamy sauce" dishes that look similar on camera but have different macros by partially swapping Greek yogurt for heavy cream. Dropping a couple tbsp olive oil into my marinara sauce does wonders for flavor but barely affects appearance when plated. Same with lard in my refried beans.