The main reason for the large catalog size is large size of all images in the catalog. You have few options to reduce a catalog size: increase compression or optimize image size. Here is how you can do it.
1. Compress catalog. Compression defines how images are optimized inside PDF document. Navigate to Catalog Designer and open Catalog Properties to change compression settings.
You should consider to apply higher compression for catalogs that have more pages and images. For example, even a small sample catalog has 4 times difference in size between Uncompressed and Best compression settings (2MB vs 0.5MB).
However, there is a trade-off: higher compression reduces image size and resolution (less details and quality on PDF zoom).
2. Optimize images by scaling down images to the necessary dimensions. You can do it using external tools like Photoshop or Image Manager cropping or resize.
Rarely you need the full page size image (around 600 x 800 pixels for PDF) unless your images has a lot of tiny details that should be well distinguished in 2x, 4x or larger zoom (maps for example). As a rule of thumb, 400 x 400 pixels image is perfect for size about quarter of the catalog page. Thumbnails could be as small as 200 x 200. Mass Image Resize keeps ratio between width and height and do not resize images that are smaller than target maximum dimensions. The minimum target size is 150 x 150.
Keep your eye on the total image size on the catalog Page tabs. Catalog Designer displays image count and size for every catalog page that allow you to find heavy pages and optimize images for them. Warning will be displayed for pages with total image size more than 500KB.