These two phrasal verbs are often used wrongly. In the English language we “fill in” (rellenar) forms (formularios, impresos). It is wrong to say (as the Americans do) “fill out” the form, although it is understood.
The Americans have difficulty with these phrasal verbs; they often say “fill out the visa forms” or “fill out the document”. Please do not make the same mistake.
If a person “fills out” he puts on weight (engorda), and the sails on Mr. Trump´s yacht can “fill out” (henchirse) if there is a strong wind.