Thunderbird gives you a few options to manage your email storage. Assume an email with both HUGE inline photos and a 20MB attachment.
For the attachments:
1. If you need the email metadata (date, time, sender) of the email, but not the actual attachment, you can right-click on only the attachment and delete it.
2. If you need the email metadata but want to keep your email cache smaller, you can detach the attachment and store it outside Thunderbird. This lets you edit, modify, trim, or store it in a compressed folder. Thunderbird remembers where you stored the attachment and keeps a link to it instead.
Advanced manipulation for inline items:
For those items encoded into the email message as b64 blobs, you can save the email to your desktop as an .eml file, open it in a text editor, and modify the SMIME entries. In some I have selectively deleted overly large photos. In others I have downloaded the photo and downsampled it, created a temporary email to store the smaller photo as a draft, then copied the smaller b64 block into the original email source. Yeah, it is a pain but I edited a 25MB email down to 2MB. I then drag the email back to Thunderbird's inbox so all the metadata is preserved but with smaller photos instead. I can delete the 25MB version and keep the smaller version instead.
I have emails dating back to 1998 stored in different folders. I copy my profile each time I move to a new PC and periodically review the email folders to delete emails that aren't worth saving. Thunderbird gives me the ability to manage my email archive better than relying on cloud-only storage.