Why is the PDF file size exported from ArcGIS Pro larger compared to ArcMap?

Answer

When exporting a map to a PDF file, the file exported from ArcGIS Pro has a larger size than the file exported from ArcMap. This is because while exporting, ArcGIS Pro maintains the vector contents in the map, whereas ArcMap rasterizes them. The exported PDF file size increases if there are more vector contents on the map. Below are a few methods to reduce the size of the PDF file when exporting in ArcGIS Pro.

Set the symbology, data, layout and the PDF Export properties

Refer to this article, How To: Reduce the size of PDFs exported from ArcMap, for suggestions on how to reduce the size of PDFs exported from ArcMap that are also applicable to ArcGIS Pro.

Generalize and simplify features

Use the Generalize or Simplify Polygon tools to reduce the size of feature classes and the exported PDF. Refer to ArcGIS Pro: An overview of the Generalization toolset and AcrGIS Pro: How Simplify Line and Simplify Polygon work for more information.

Note: 
Esri recommends creating a copy of the feature classes before running the tools.

Check the 'Output as Image' option
In ArcGIS Pro, share a map or layout as a vector format (EMF, EPS, PDF, SVG, SVGZ) and check the Output as Image option. Selecting this option reduces the output file size.