Actually they were a lot lighter than you'd think for their size, they were FULL of air sacs specifically to make them lighter. They moved by having really, really straight legs so the weight was transferred straight down into the ground, and they walked on cushioned pads just like an elephant. Their lungs were sort of constantly cycling air around because otherwise it wouldn't be much good, and the whole point of having a really really long neck was specifically that they didn't need to move much!!!!
Like, they're incredibly unspecialised herbivores who just ate everything and let the gut handle it (hence why they had to get so big, just because they had to fit A LOT of gut in there) like their mouths were basically just Rakes.
This thing was built to sweep its head from high to low to near to far and rake in everything it could eat. By evolving a long neck it basically meant it was giving itself a huge range of motion to eat with but also being VERY energy-efficient because moving your legs takes a LOT more energy than just. Tilting your neck up and down, especially at that size. So this thing was basically built to be incredibly, INCREDIBLY energy efficient, light and weight-spreading for its size.
Blue whales sort of practice a similar feeding style, where because they're so Big and need so Much food, they just sort of went "ok im just going to take in AS MUCH food as possible as efficiently as possible" and the thing is. Using baleen takes 0 effort really other than opening and closing your mouth. Which is why whales can grow to be really big. See in the ocean weight matters less, so you can afford to just have a big mouth, but land animals have to do it differently.
Being Big isn't just a case of having lots of food available. It's about being as Efficiently able to eat it as possible. Hell, i mean elephants basically evolved the same thing, except they've just given themselves really long noses rather than necks. It means the elephant can stay in one spot while still reaching huge amounts.
Idk if you actually wanted answers, but, well, here they are.
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