Well, as you said your workbook has lots of data, so the converted PDF file would be of larger sized. We don’t provide any zip type utility (in the shape of APIs) to minimize the generated PDF file’s size. By the way, you got somehow larger PDF file in size, the reason is simple, Other PDF tools like Adobe Tools adjust the pictures resolution or for some other objects (if you have in the workbook) to a lower level, but, we will retain the original resolution related to all the objects including pictures, so, the size might be a bit larger.
Moreover, Aspose.Cells for .NET is a spreadsheet management library rather than PDF management engine, this is our additional feature (Excel to PDF) for the users.
If you still find that the size of the generated PDF file is a bit large, kindly give us your template file, we will look into the issue soon.