During a queue run, sendmail holds information in memory about all the files being processed. It does this so that it can sort them by priority for delivery. Beginning with V8.7 sendmail, there is no limit (other than consuming all memory, or setting the MaxQueueRunSize option, MaxQueueRunSize) on how many queued messages can be processed during any queue run. Prior to V8.7, that number was fixed by the constant QUEUESIZE. QUEUESIZE has been retired and replaced with QUEUESEGSIZE, which is defined in sendmail/conf.h as:

# define QUEUESEGSIZE   1000            /* increment for queue size */

It should be changed only if your queue continually contains a huge number of messages. If you notice many messages such as this being logged:

grew WorkList for...

you might need to modify QUEUESEGSIZE. Doing so requires that you edit sendmail/conf.h and recompile.

QUEUESEGSIZE can be traced with the -d41 debugging switch (-d41.1).