Bandwidth enhancement for an antenna over AMC

Hello
I will really appreciate if someone could answer my following query.

"I have designed and measured a coplanar planar printed monopole antenna over an AMC surface. I get improved bandwidth when I increase the array size for the AMC, for example, I get better bandwidth for a 8x8 AMC than a 4x4 AMC. Is there any explanation for this phenomenon. Thanks.

Sorry that I am unable to understand.
Query: same size for both 8x8 and 4x4?

yes unit cell size is same for both 4x4 and 8x8 arrays.

BW is related to volume.
when you want to compare 4x4 to 8x8 then use same area of 8x8 keeping the rest as blank.
Unit cell remains same , volume also same then BW must match. This is my point.

so you are saying that it is the volume of the AMC that is responsible for increased bandwidth.

Yes , correct