A quick test on a database with 187 medium-length scholarly references showed a reduction in size from 1,671,168 bytes to 1,306,624.
Here's an article with some pros & cons:
Access File Formats: ACCDB vs MDB
They point out with .accdb's, you can:
- Include attachments in your database
- Use multivalued fields
- Allow SharePoint and Outlook to trust these files, due to "[i]mprovements in the database security model [which] allow for the security validation of database files"
- Have supposed cryptography improvements due to use of the Windows Cryptographic API
Here's a M$ article that tells how to do attachments as database record fields:
Attach files and graphics to the records in your database
The first article also says: "There are also two limitations that you should consider when using ACCDB. ACCDB databases do not support user-level security or replication." I made a copy of a database, and it opened, no problem - even in a different directory; so I don't know what you are missing out on in terms of replication.
Note: I was unable to Save As to convert it to a .accdb file, as given in the first article. I did File | Save & Publish, and it let me.