Although the Portuguese word maior looks like the Spanish word mayor, the two words often do not mean the same thing. In Portuguese, maior refers to size and is equivalent to más grande in Spanish.
English: This stadium is bigger than that one.
Spanish: Este estadio es más grande que aquel.
Portuguese: Este estádio é maior do que aquele.
But if you are referring to someone who is older be sure to use mais velho in Portuguese.
English: The older gentlemen should sit down.
Spanish: Los señores mayores deben sentarse.
Portuguese: Os senhores mais velhos devem se sentar.