Photoshop Extendscript: get active document weight?

alert(px * thisDoc.resolution * bits / 1024 / 1000 / 1000);

It seems that you have confused the units of data transfer rate and data storage units.

1 mb = 1024 kb

1 kb = 1024 bytes

1 byte = 8 bits

doc size in bytes = px width * px height * number of channels * bit depth 

thisDoc = app.activeDocument;

// ----------- get the number of pixels
var savedUnitType = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;
var px = thisDoc.width.value * thisDoc.height.value;
app.preferences.rulerUnits = savedUnitType;

// ----------- get the number of bits/pixel
var bits = getDocBits(thisDoc);
// ----------- 
alert(px * bits * thisDoc.activeChannels.length / 1024 / 1024/ 8 ); // Mb

function getDocBits(thisDoc) {
	var bits = thisDoc.bitsPerChannel;
	if (bits == BitsPerChannelType.EIGHT) {
		return 8;
	}else if (bits == BitsPerChannelType.ONE) {
		return 1;
	}else if (bits == BitsPerChannelType.SIXTEEN) {
		return 16;
	}else if (bits == BitsPerChannelType.THIRTYTWO) {
		return 32;
	}
}