Friend of mine moved from South Korea to the US and within 6 months had gained a noticeable amount of weight, we discussed it and I said "just gotta watch the calories bud" - confused face - he didn't know how to actually go about calorie counting, he didn't know the calories in 1lb of fat (~3,500), he'd never had to do it in Korea.
Its impossible to not get blindsided by HFCS and the general sugar content of American foods if you compare the retail shelves like for like with Europe.
In Ireland and other European countries they literally have to 'cover up' the Breakfast Cereals in the 'American Speciality/Import' section due to their misleading nutritional information and/or child targeting.
For example, American formulation PopTarts have to legally cover up the claim on the front that they are "a good source of nutrition" and another sticker on the back with a factual nutritional breakdown, as is mandated by most EU members.
The McMuffin is pretty much the most balanced meal at McDonalds! If you wanted to quibble over it, you could make it with a poached egg white instead of a whole egg, and dress it with olive oil instead of butter, to cut down on the cholesterol without removing too much energy. At ~300 calories a McMuffin is hardly sufficient for many people to get through the morning.
Tbf the Bacon McMuffin is a lot better than I thought it would be. The 'healthiest' option other than a pure salad in Irish McDonalds would be the grilled chicken wrap they have - "The BBQ and Bacon Chicken One - Grilled"
As for your 300 calories contention - 300 calories of scrambled eggs or steel cut oats would do me fine to get through a morning. 300 calories of McNuggets would not conversely. I'd imagine most of that has to do with satiating fibre, protein and fat therein.