Why is the CG envelope wider when the aircraft is heavy?
So if I understand that correctly the CG can be more forward when the aircarft is heavy.

Not really, although I guess you could say the index can move &quot;forward&quot; because of the way the form is drawn, but the index is not the CG. Indices are a composite number, namely the moment divided by a reduction factor.

The use of index numbers and a reduction factor greatly simplifies
weight and balance calculations, especially for large aircraft. The
index is a moment divided by a reduction factor and may be found by
this formula:
Index = (Weight x Arm) / Reduction factor
(from http://avstop.com/ac/weightbalance/ch4.html)

While indices make manual calculations easier than using %MAC and weight (the way computerized systems typically do it), they do tend to obscure things in my opinion.
If you want to see a 747's weight and balance envelopes using %MAC and weight, go to 747.terryliittschwager.com and select any 747. The top one on the aircraft list will do, N402YY. When the aircraft comes up, scroll down to OPERATING ENVELOPES.
You can position your pointer over the vertices to see their values. Note that the forward limit for takeoff at 580000 lbs is 11.0 %MAC, staying at that limit up to 778000. Getting heavier from there moves the forward limit aft, not forward, and additional weight from there really sends it aft.
The forward limit for the zero fuel and landing envelopes stays constant throughout the weights of their range.