According to the DASH specification, codecs, audioSampleRate, frameRate and AudioChannelConfiguration are all allowed to appear at the AdaptationSet level not just the Representation one. We can use that to slightly reduce the size of the manifests by specifying it on the AdaptationSet, instead of each Representation, if the value would be identical for every Representation in that AdaptationSet.

Examples of situations that can be useful in:

1. The representations in WebM adaptation sets tend to all have the same codec e.g. opus, vp9 and vp9.2 (HDR), so it makes sense to hoist the codecs attribute
2. For 4k+ 60fps HDR videos the vp9.2 streams all tend to keep the original 60fps frame rates, so it makes sense to hoist the frameRate attribute

This doesn't massively reduce the size of the manifests, as the majority of the size is occupied by YouTube's very long streaming URLs. I do have an idea I want to try though, having a BaseURL element at the AdaptationSet level with a common prefix for the URLs in the BaseURLs in the Represenation, this will need lots of experimentation, as developer specified URLTransformer function could produce all sorts of wacky URLs. Another thing to watch out for is OTF formats, as they use SegmentTemplates instead of specifying a BaseURL but would still be affected by BaseURL elements on the AdaptationSet.
TL;DR: The streaming URLs are the biggest contributor to the manifest size, I have an idea to shrink them but will need a lot of experimentation so it doesn't break anything.