[Dovecot] doveadm altmove fails to pack messages?

Hello again.

When I use doveadm altmove, I notice that it may result in some very
small storage files in either the primary or the alternate storage
location. I'm not quite sure, but it almost seems like every storage
file that contains messages to be moved is simply split into two
parts, even if a part might only contain a single message.

This seems quite curious, since the idea of mdbox is after all to
store as many messages in a single file as fits in mdbox_rotate_size,
and dovecot-lda can quite successfully append messages to existing
storage files. So why can't doveadm altmove? Is this the expected
behavior, or is there something wrong with my setup?

Thanks again,

Lauri