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I think that's a tricky one when you consider the potential epochs of time. Many have hypothesised that the g force affecting the planet has changed over millions of years and that in the past gravity may have been weaker allowing all creatures on earth to grow much bigger. You can see that with massive fossilised dragonflies which couldn't physically fly with our current gravitational pull. With that said if these people were much bigger and gravity was weaker then building a dolmen would be simple.
Cheers for reading and commenting Matt.