MAM message (bundle) length limit 638036 Bytes, bug or by design?

[zhengxiaochen]

Hi, is there a limit for the length of 'trytes' in MAM.create(mamState, trytes)?
I encoded a string to str Trytes with trytes = iota.utils.toTrytes(str) , then passed the encoded 'trytes' to the MAM.create() to create a message. It worked well with a string less than 638036 Bytes, but caused the following error with even one more byte (638037).

fatal runtime error: allocator memory exhausted
trap!
trap!
abort("trap!"). Build with -s ASSERTIONS=1 for more info.

Is this a bug?