Why smaller code size with XEN on ARM?

This white paper (Xen ARM with Virtualization Extensions whitepaper) indicates that "Xen on ARM is 1/6 of the code size of x86_64 Xen, while still providing a similar level of features". what does this mean? Does this mean that Xen/ARM is better than Xen/x86? We cannot simply just make this conclusion, but anyway smaller code size means smaller TCB, which can reduce security risks (e.g. security vulnerabilities). 

ARM TrustZone (Security Extension) and Virtualization Extension vs x86 Virtualization Technology

A typical virtualization system on both x86 and ARM includes three major parts:

• CPU virtualization
• Memory virtualization, and 
• I/O virtualization (device, interrupt virtualization). 

Exploit NULL pointer dereference bug (ARM and x86)

A NULL pointer dereference occurs when a pointer with the value of 0 (NULL) is assumed to be a valid memory location, and that pointer is then accessed. A write from, or read to, the memory address 0x0 (+ small-offset) will generally reference invalid or unused memory, which typically leads to the majority of published denial-of-service (DoS) attacks both locally and remotely.

Introduction to Processor Hardware Security Features in x86 & ARM Architectures

x86 and ARM processors both provide many hardware enforced security features, e.g. NX (No-eXecute) for executable space protection, to help system software engineers to build a secure computing environment. 

This article summaries those security features for both x86/Intel and ARM architectures, and explains how are they used by Operating System.