Is there a limit on JSON file size?

I'm getting error -400 from makeWebRequest on a biggish JSON data that is well formatted.
It's a dictionary with three arrays and each array loads fine individually and as pairs,
But, through trial'n'error, it will load as 3 arrays when the third array is constrained to 5 elements, but crashes with six. (ultimately it will have ~20).
Each additional element of the third array increases Peak Memory by 200 bytes.
There's nothing ugly about the sixth element as I have swapped it with the 5th OK.
It looks like I'm hitting a file size constraint, even though I'm well inside the memory limit of the app: with Peak Memory at 94Kb.
The physical file is around 7Kb. It loads when 7,019 bytes but crashes at 7,075 bytes.

Is there a physical limit to JSON files?