I want to understand reasons behind the magnitude of difference of NAV Regular Vs Direct plan

The difference in NAVs of regular plans and direct plans are due to expense ratio only. Structure of expenses are different for different AMCs, as you yourself have observed by comparing the expense ratios of Franklin India Bluechip direct plan and HDFC Top 200 direct plan. When you compare the regular plans and direct plans, instead of comparing NAVs, you should compare the difference in expense ratios. The difference in expense ratios between the regular plan and direct plan of HDFC top 200 fund is 0.6%, whereas that for Franklin India Bluechip fund is 0.8%. That explains why the difference in NAVs of regular and direct plans is higher for Franklin India Bluechip compared to HDFC top 200. Comparing NAVs can be a little misleading, as you would have realized.