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"
Roughly the same calories as the McMuffin but half the fat, twice the fibre, and 50% more protein.
https://www.mcdonalds.com/ie/en-ie/good-to-know/nutrition-ca...
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.