that depends what you mean by volume.
From the teacher's perspective, they are doing exactly that, giving 20 minutes homework, which is the 'same' amount
I do understand what you are saying, but for example in my ds class (Y6) he is doing something like this:
write the opening chapter to a story, concentrating on setting the scheme, introducing the characters, remember use of adjectives, interesting sentence openers etc etc
They would write an A4 side of paper, a piece of complex writing. It would take ds about 20-30 minutes once he had stopped prevaricating and actually sat down and did it
the middle group might have same task, but only be expected to produce one/two paragraphs, or be given the same task in a much more structured format, like answering questions.
the bottom group in his class could not do that task, no matter how long they spent working on it. In fact I happen to know that there is one child in his class who is working at a Y1 level on reading and writing.
The tasks cannot be the same because not all the children are able to do the same tasks.
Simlarly in maths, you cannot ask a child to do complex multiplication/division questions when conceptually they are not yet able to do long multiplication.