macOS Ventura 13.2 Differences between ~/Library/Containers/Mail/Data AND ~/Library/Mail

I've migrated my Mac probably from Tiger (maybe clean installed once in between), so the Mail folder structure is probably a bit messy. Nevertheless, I just got a new mac and migrated with Migration Assistant again and found my new Mac has much less space left than the previous one. Long story short, I ended up deleted quite a lot of mail attachment from ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads. According to my search, it should be safe to remove them (right?). 20GB space recovered. I've tried rebuild the mailbox with Download Attachment "None" selected and that 20GB was still untouched...Anyhow, a lot of time wasted. My question now is it seems there are similar sized V10 folders in ~/Library/Containers/com.apple.mail/Data/Library/Mail AND ~/Library/Mail I see some posts online said the /Container is a sandbox and it's a link to other places, but is it normal to take similar amount of space? In my case, ~/Library/Containers/com.apple.mail/Data/Library/Mail took 51.5 GB (I deleted the 20GB Mail Downloads from here) ~/Library/Mail took 81.8 GB The structures are not exactly the same but some folder names in V10 (they both have V10) are exactly the same. Is it normal for Mac Mail to use double the disk space? Or it's the migration left over? If it is normal, any other mail client can perform as-good search feature and take much less space? If it's not normal, what might be the best course of action to keep the Mail disk usage under control? If I can confirm it's not normal, I'll be happy to remove all accounts (and remove all the mails/attachment, don't know how atm though) then redownload them from the IMAP/Exchange servers. I "think" they are still all kept on the servers. BTW, the Apple chat and phone support is useless in my experiences... Thanks!