That is not an easy question to answer. Behemoth is an evocative word term for something of monstrous size, power, or appearance. It is not defined by precise quantities but is something that you know to be huge when you see it. It is frequently used to describe large telephoto lenses. A quick search finds that the 300-600 has already been defined as a behemoth by a respected reviewer.
https://uk.pcmag.com/lenses/158089/sigma-300-600mm-f4-dg-os-sports
“After two weeks of lugging the Sigma 300-600mm F4 around for wildlife snaps at a refuge, the zoo, and my backyard, I walked away with some fantastic photos and a seriously sore shoulder. At 18.5 by 6.6 inches (HD) without the hood attached, and 8.8 pounds, the lens is a behemoth. That's big even among its peers.” And the reviewer recommends: "Keep your Chiropractor on Speed Dial"
So, I think that if there were a hypothetical line to be drawn, the Sigma is likely to be on the behemoth side. Though this guy might disagree.
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