Doesn't sound like overloading is the problem--a typical SPF 6 x 6 should be good for at least a compressive stress of 875 psi, meaning a total load of more than 26,000 pounds. I suspect your contractor either used green wood, or possibly the side exposed to weather is sucking in excessive moisture (causing it to swell and grow), or is expanding more than the shaded interior face when hit with direct sunlight, also causing it to lengthen and bow outward.
Let your contractor propose how he plans to fix his problem, and then let us know here what he says, giving us a chance to give you some additional background or options.
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