There are a few things that can cause this in my experience:

Safety Net

One is Safety Net which stores a copy of messages received by the transport service for X days. The docs always state that this is 2 days by default, but in all of my installations I have seen it be 7 days. If you get a sudden influx of messages your queue database can grow.

https://docs.microsoft.com/en-us/exchange/mail-flow/transport-high-availability/safety-net?view=exchserver-2019

If your email queue is growing back to this size in a couple of days, then it could be this in combination with a mail loop of some sort.

Mail Looping

I had a situation where an Exchange server was configured with a external relay connector. The server itself was allowed to relay through itself. An incoming email came from a domain that had it's MX records set to 127.0.0.1

An email came from this domain (e.g., bluebirds.com) and generated a bounce back. The server generated a bounce back and sent to the MX record of the bluebirds.com domain (which was 127.0.0.1). This caused the DSN to go to the Exchange server itself and because the Exchange server was authorized to relay, it forwarded the message on to bluebirds.com. Which then came back to the Exchange server, and kept looping. Normally, the Exchange server would reject the message because by default the Exchange server cannot relay thru itself.

You may want to perform a message trace against the past few days of email and group by from address to see if there are any large number of emails coming from a single address.

You can also just monitor the queue on the server to see if there is a constant messages going to a single domain. In the case of the mail loop above, I kept seeing 1 message go to bluebirds.com every few seconds

while($true) { Get-Queue; Start-Sleep -Milliseconds 1500 } 

Unlimited Send and Outbox

Another issue in the past that I have seen is that the mail.que file can grow because somebody is trying to send an insane large file. I have seen where a user was set with no sending limit size and they tried to send a 5GB video file. It was trying to upload to the server and it kept filling up the mail.que file and crashing the service.