After creating a new replica of unused a db in lotus notes there is difference in db size. But the no of documents are same

Any idea of what might be the reason and how to resolve it. This is in Version8.5

I tried deleting the replica and creating again but it is same. No of documents are same in replica and in the main db. Is there any mistake while creating new replica? If yes please provide solution.

The size difference is most likely because of the size of the view indexes in the original db. The view indexes in the new replica have not yet been built

First of all: No, this is not an error. As long as the document count is identical everything is ok.

Per Henrik Lausten already gave one reason for this difference but it is not the only one. Here are some more possible reasons (repeating the one of Per just for completeness):

View indexes
They are (by default) only built when views are used. Views with sortable columns even have more than one index to be built. They are often the reason for the biggest differences in size. You can check the size of view indexes in admin- client with right-click on the database, there is a point to check indexes.

White space
Whenever anything in a database is deleted the physical size of the database does not instantly shrink. The deleted stuff leaves "white space" in the database that can be removed by compacting the database. A new replica does not contain white space. The white space can be checked using the "Percent used" button in the database properties

Deletions
Soft deletions
Soft deleted Documents in the trash can are usually not replicated

Deletion Stubs
Somehow related to white space: In order to prevent deleted documents from "coming back" due to replication Domino keeps stubs of deleted documents for (by default) 90 days in a database before it finally purges them. These deletion stubs are small but can sum up if you do a lot of deletion in a database
On disk structure (ODS)
Depending on how old the database is and how the settings on the server are the databases might have different on disk structures. Almost every version brings its own ODS

• Prior to Domino 9: ODS <= 43 (Templates ODS 23)
• Domino 9: ODS 52
• Domino 10/11: ODS 53
• Domino 12: ODS 55

Every ODS has different efficiency in storing data and therefor needs different amounts of space for the same documents.

Related to ODS there is the type of compression of attachments within the database. It used to be "Huffman" but in all recent versions you should create LZ1 in the database properties (Option "Use LZ1 Compression for Attachments").
Of course this also causes different sizes on disk.

Internal Database structure
The smallest factor for size differences are internal tables for the functionality. E.g. there is the "UNC-Table" that keeps track about all fields and their types within a database. Unless you do a lot of changes in designer and have a lot of fields this will not be a big difference. The UNC table is rebuilt for every replica and might be smaller for new replicas as it does not contain the complete "history" of the database

DAOS
Different replicas can have different settings for using DAOS. This of course also impacts the size of the database itself.