As far as politeness is concerned, the woman who wrote the Miss Manners newspaper column back in the 80s once said a good rule of thumb is to only tease people about the qualities that they are proud of and fond of.
This can take a bit of discernment, but isn’t rocket science.
The fact that allie o would take it as a compliment to be called a beast, probably indicates she likes that about herself, that she is tough, that she is a fierce competitor.
Was steve prefontaine secretly fond of his chunkier size? If no, then you would not want to say it in conversation. Bringing up a sore topic to someone (under the guise of joke or whatever), is a dig — rude.
Having said that, a reporter or commentator is kinda worthless if they cannot point the harsher truths at play. If someone is running a bad race, say that. If someone’s form is falling apart, mention it. Reporting and commentating should not follow rules of social politeness.
If someone is not at race weight, it is fair game — not because it makes them look prepubescent (allie o at her thinner weight) or because it makes them look too fleshy for your aesthetic liking, but because a person too far above or below race weight, cannot perform as well.