[services] Backup - Fix data size database limit

Summary:
Depends on D4232

The database limit is invalid. I found out in the docs that it should be 400kb (link in the code).

Test Plan: -

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: varun, ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4233