enhance: add mem size for index file and fallback to multiply with serialized size

[chyezh]

issue: #38715

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.33%. Comparing base (5304707) to head (6ed14c8).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #38716   +/-   ##
=======================================
  Coverage   69.33%   69.33%           
=======================================
  Files         292      292           
  Lines       26187    26187           
=======================================
  Hits        18158    18158           
  Misses       8029     8029           

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	69.33% <ø> (ø)
Go	∅ <ø> (∅)

[tedxu]

/lgtm