What is the purpose of this asymmetric IC footprint?
Care has to be taken at 18 ghz. Compromises and shortcuts taken at for example, L-band, are not acceptable at higher frequencies. 

The grouping by function reflects the internal construction of the chip and the manufacturers attempt to improve isolation between the switch ports, which is a very important spec item for an RF switch. 

The RF ports will be carefully simulated to give the correct impedance, and not require large pads which could cause unwanted capacitance to ground. 

Ground pads will be enormous to allow lots of vias to be placed directly underneath the chip to make as short connection as possible to your ground plane. 

You are the designer, if you are not interested in correct operation at 18Ghz then you can likely take some shortcuts. Let's call it a calculated risk.