I am using iphone 6 ios 9.3.3 my battery Max capacity is 1118 mAH and design capacity is 1751 ?
Why its Max capacity is is not 1751 what to do to fix it.
Your battery is worn out
Batteries, particularly lithium-ion batteries loose the ability to hold charge as they age. The biggest indicator of how old a battery is is not just the age but the charge cycle count, which is the number of time the battery is charged to 100% and discharged again back to 0. Note that this doesn't have to happen all in one charge but is cumulative.
645 charge cycles is a lot. The current estimate of the lifespan of an iPhone battery is 300 - 400 charge cycles There is nothing you can really do to fix this except use the iPhone in such a way to maximise the remaining life of the battery. More information can be found here.