Assessing any optional subject with length of syllabus is not the right criterion for optional subject selection for Civil Services (Main) Examination.
You should pick that optional subject that matches with your aptitude. Your liking and interest in the subject would help you in any optional subject even if it has a lengthy syllabus size.
By the way, both the optional subjects shortlisted by you are among the popular subject candidates pick. We suggest you to have a look at the syllabus and previous years’ question papers to understand the level and current trends and based on this, take a rational decision.