jsfWriteFile seems to get the wrong dLen(data length)

jsfWriteFile seems to get the wrong dLen(data length), when i enter the code below, i have to encode the data with base64, for some reason(utf8 multibyte Symbol or characters ), some times the data encoded is larger than befor, as use atob(...) when writing to the Storage. for example, the data original is 1000 byte, base64 encoded is 1500 byte, so the question is if i use require('Storage').write("abc.js",atob("­Cmcu...DApOw=="),0,1000); error returned:
Uncaught Error: Too much data for file size at line 1 col 5563

use 'require('Storage').write("abc.js",atob(­"Cmcu...DApOw=="),0,1500);' worked fine, but the storage space is wasted a lot ... the final data after base64 decoded just needs 1000 byte :(