I am trying to understand the mailbox size.

fedoracore What you see on ECP, whatever version of Exchange or Exchange online if you are using includes all data in IPM except Recoverable items ( deleted items in this case ) whch is shared across your IPM and non-ipm data folders.

IPM contains the default front end folders like inbox, sent items drafts , etc and deleted items (partially) , when an item is deleted from deleted items and it moves to recoverable items folder (Last chance) it moves into NON-ipm which is separate from Mailbox regular statistics and hence displays the same on ECP/Admin Center which is different from your Outlook statistics.

For Best results, always run below command to fetch statistics which will give you exact data a mailbox is consuming Technically from backend mailbox database.

Get-MailboxStatistics user@domain.com | FL 

Also for getting information about each specific folder and how much data your recoverable items folders are consuming - 

Get-MailboxFolderStatistics user@domain.com | Select Name , Foldersize , Itemsinfolder

For specific to Recoverable items run below - 

Get-MailboxFolderStatistics user@domain.com -FolderScope Recoverableitems | Select Name , Foldersize , Itemsinfolder

https://docs.microsoft.com/en-us/exchange/policy-and-compliance/recoverable-items-folder/clean-up-deleted-items?view=exchserver-2019

https://docs.microsoft.com/en-us/exchange/security-and-compliance/recoverable-items-folder/recoverable-items-folder

https://docs.microsoft.com/en-us/exchange/policy-and-compliance/recoverable-items-folder/recoverable-items-folder?view=exchserver-2019

Cheers !!

Ankit Shukla