120kg man given the same dose as a 40kg

I'm reading the instruction manual for Osmivir, from Royce, Malaysia
that is is equivalent to Tamilflu, or Oseltamivir phospahae.

Adults and adolescents 13 year of age and older, is 75mg twice daily
for 5 days.

My son is older than 13 years although weighs at 39kg and he prefers
pills instead of syrup that is too messy for me to administer.

If you weigh 120kg, i.e. obese, you'll be given the same dose.

A very important message is that treatment for influenza should be no
later than 48 hours after the onset of the initial symptoms of
infection.

Note that it is for all types of influenza, except that for Swine Flu,
it is 100 times life saver compared to normal flu that Tamilflu only
relieves symptoms from 3 days to just 1.5 days. For Swine flu,
Tamilflu can be a 100 times life saver indeed, compared to normal flu.