...do que o meu gato... are all the words necessary? How about ...maior que (o) meu gato...?
In this example of comparison you can omit (do) as well as "o"
Does 'maior' ever mean 'older' like in Spanish? Or it's strictly 'bigger' in portuguese?
No. Mayor = mais velho. Más grande = maior. Take care. It's tricky. Its wrong to say mais grande in Portuguese.
Actually, It depends on the context. For example:
"My younger brother = meu irmão mais novo" but you could also say "meu irmão menor (as in English "my little brother")."
"My older brother = meu irmão mais velho" and you could also say "Meu irmão maior" (in English "my big brother". It's very common when you're asked about their ages. For example:
Quantos anos têm seus irmãos? / How old are your brothers (siblings)?
O maior (mais velho) tem 18 e o menor (mais novo) tem 10 / The oldest is 18yo and the youngest is 10yo.
Since maior is in the comparative degree, are do and que absolutely necessary? If so, why?