Why %RSD of execution times, while sorting hundreds of arrays, is lower for larger arrays of random integers?
In the case of insertsort, the complexity (time) depends on the number of swaps performed during sorting. A kth element (k=1,2,3,...,n) in an array of size n, may require 0 to k swappings. 

The execution time mentioned in the question depends on the sum of all the swaps performed during the sorting of a complete array. 

Considering the random initialization of the arrays, the number of swaps performed for any kth element would be a random number bounded by the given range. 

Now, If we consider the average value of the number of swaps per element (sum of all the swaps divided by the number of elements in the array), then, according to the law of large numbers, as the number of elements per array will increase this average will get closer to the expected value.

i.e., the variation in, the total number of swaps per array, among different arrays, will decrease. 

Hence, the change (%RSD) in the execution times will also decrease. 

--to be continued--