In my world dragons are supposed to be an apocalyptic terror upon the world. Even as baby’s they’re a serious threat to a town. So it feels weird to have them be smaller than a horse. So I’m debating just making baby’s large, young huge, https://omegle****/  

As a follow up question. What would the consequences of adding a new size since adult and ancient would end up being the same size?

I think the consequences would be fairly minimal. Assuming you increase reach as well where it makes sense, it's really just about how much space it takes up and controls. This could have different impacts on an encounter based on the environment, but assuming you aren't using a map made for a smaller creature I think it would be fine.

As for the second question, note that Gargantuan size is 20 feet or more. The tarrasque is like 100 feet long or something. So you can still have ancients be larger in terms of how much space they take up even though they would be in the same size category as adults.