Well, I am sorry you feel that way, but I see it the other way (obviously).
I find it ridiculous that a T-rex, for example, has a 4d12 bite as huge creature but an ancient dragon (gargantuan, which presumably a larger head and bite-size) only does 2d10.
The symmetry is simply a nice way of expressing the difference between a large dragon's 2d10 and a gargantuan dragon's 4d10. It is NOT symmetry for the sake of symmetry as you seem to think. Yeah, I wouldn't do that unless the values happened to jive with the in-world reality as I see it. The dragon is getting larger. Larger teeth, larger bite size (mouth), etc. It makes sense the base damage for the bite should go up, but it doesn't.... The same goes for the increasing fire damage that rides the bite damage. It goes up finally to 4d6, but the 1,1,2,4 progression (while ok) would not represent the increase in the power of the dragon breath as well as a 1,2,3,4 progression.
In the same way, the breath weapon doing 6d6, 12d6, 18d6 and 24d6 would work well IMO instead of the values RAW of 7d6, 16d6, 18d6, 26d6. Most are ok, but man that 16d6 for a young red dragon is just "out of place" compared to the other values (which are close to the progression I would do personally). I mean, especially when you consider the fire rider damage between a young adult and an adult. The young adult is 1d6 but the adult is 2d6, yet the young adult's breath weapon nearly matches the power (i.e. damage) of an adult? No, that just doesn't sit well with me in the narrative sense, either.
Maybe, but I am just going by the sheer size increases of the dragon as well. If the progression was more about die size instead of number, that would at least make sense. Maybe d6, d8, d10, d12 or something instead of 1d10, 2d10, 2d10, 2d10... It is like they are suggesting the dragon's head/mouth never gets larger despite its body size increasing. Maybe that is their fiction? If so, I can't agree with it...