According to documentation of Google cast, Starting with version 4.3.1, Cocoapods uses static libraries which is a ~200MB download and a ~600MB framework.
There is also a dynamic sdk that is ~20MB download and ~50MB framework.
This repo hosts the dynamic library in Cocoapods which for some unknown reason google haven't done.
Specify it in your Podfile:
pod 'google-cast-sdk-dynamic-ios'