Lightroom Catalogs are saving at 500MB+

My Lightroom catalogs are all saving as 500MB+ all of a sudden.

I am a LR editor for photographers and when I download my clients catalogs, initially their file sizes look correct for the amount of images they have but, when I open the catalog in my Lightroom, the catalog size massively increases straight away to over 500MB. As one example, I have a catalog and smart previews with only 4 images in it but, when I opened it in my Lightroom, the Catalog file size increased to 500MB. 

Does anyone know why it's doing this suddenly or knows how to fix it? 

Thanks!

I'm using Lightroom Classic CC 8.3.1 version with Mojave OS.