How do I decrease the size of the pcap file?

Please check the port numbers that are seeing most of the packets. These may not be related to your task. Filter them out during packet capture. Alternatively, you can filter them out and create a smaller pcap retaining only the necessary packets using wireshark.

Refer the cheatsheet for hints as well https://docs.google.com/document/d/1f74oP50L11fjmzcdTJ2dlc8qimgWbBbh8aC45YbXkYc/edit?usp=sharing