Some confusion about Qt Canvas3D

BriFuture

I'm using Qt Canvas3D module to draw a 3D scene,

when I use

var size = 1024;
gl.bindBuffer(gl.ARRAY_BUFFER, mybuffer) ;
gl.bufferData(gl.ARRAY_BUFFER, 4*size, gl.DYNAMIC_DRAW);

gl.bufferSubData(gl.ARRAY_BUFFER, offset, myArrayData);

this segment of code works well.
but when I set the variable size to 8192 or larger, it will be something wrong and show me a very strange graphics, and then it won't show ever.

But sometimes, I set size to 8192, it may work as my expectation. I don't know what's wrong with the code.

Is my offset wrong or the the function bufferData(glEnums target, long size, glEnums usage) I use wrong?

and if I use a new Float32Array() as the second argument, it will work very normally, but I dont know how to add more vertices into the buffer, can anyone help me?