Database Files of Differing Sizes
This warning states that a database data file is a different size to the other data files in the same filegroup. To ensure proportional fill between the data files and, therefore optimum performance, all data files in the same filegroup should ideally be of equal size with similar free space.
Set all data files that are within the same filegroup to the have the same size and autogrowth settings.
Any suggestion from Aireforge should be thoroughly tested before being implemented into production.
Additional Data Files and Proportional Fill | SQL Server Central