How The effective channel width of the transistor is affected by the “bird peak†.

[MubarakKhan]

I'm reading "CMOS_Transistor_Layout_KungFu (http://www.eda-utilities.com/CMOS_Transistor_Layout_KungFu.pdf)" document in that he explains how isolation effect channel width of MOS transistor in page -14 to 15.But what i understand is it will effect the diffusion area and channel length. i didn't understand how isolation effect width of transistor.