Reducing image size in RSS only

It’s very possible, but quite hard to accomplish reliably for all cases (I was working on a plugin for client to do this and more, but sadly the project was never completed).

But if you need this for specific blog that you have control over – then task can be simplified with some constrains – mainly limiting processing to attachments only.

If so:

• filter the_content_rss, extract all images with regexp;
• determine attachments IDs for images, see this question and my answer there for some helpful code;
• retrieve attachment img tag in smaller size and replace currently used version with it.