Thank you Dmitry.

Here are the results of gstat:

FLAT_DATA (290)
    Primary pointer page: 5239142, Index root page: 5239147
    Average record length: 213.90, total records: 8288203
    Average version length: 0.00, total versions: 0, max versions: 0
    Data pages: 533746, data page slots: 533746, average fill: 88%
    Fill distribution:
     0 - 19% = 0
    20 - 39% = 0
    40 - 59% = 0
    60 - 79% = 30
    80 - 99% = 533716
   
DETAILS_DATA (262)
    Primary pointer page: 842, Index root page: 843
    Average record length: 20.19, total records: 164512578
    Average version length: 0.00, total versions: 0, max versions: 0
    Data pages: 2455600, data page slots: 2455600, average fill: 61%
    Fill distribution:
     0 - 19% = 0
    20 - 39% = 1
    40 - 59% = 0
    60 - 79% = 2455599
    80 - 99% = 0

I guess this confirms your statement about record header size?

FLAT_DATA record length is 213.90 which is over 10 times more than DETAILS_DATA record length 20.19.
However in DETAILS_DATA we have 164512578 records which is 19.8 times more than in FLAT_DATA.

Best regards.