There’s a long standing idea tha one should make an effort to trim out text from the original which is not germane to the new content in your reply. This is not just a bandwidth thing, it’s also about decreasing the effort required for the readers to parse the message – to locate the new text and refresh their memory of the relevant bits of the conversation. Unfortunately it seems that more and more people aren’t doing the cutting.
This causes issues for me mainly because I do a reasonable amount of my mail reading using my phone. It’s no fun wading through pages of diff on an undersized screen such as a mobile phone when the “content” you’re looking for is a one line comment somewhere in the middle. Even on a full size screen it’s often difficult to locate a small piece of new text, but there it needs a much bigger haystack to be an issue.
Please, if you’re one of the people who do this have pity on those of us Reading your messages on smaller devices!