Both Unison and DSPnano offer an ultra tiny RTOS size from any perspective.  Consider the possible comparisons from one to one comparisons to proprietary embedded RTOS solutions or single loops of control which deal with the same level of complexity to get a true perspective.

One to One Comparison

There is no real one to one comparison for Unison but there is some close comparisons and some would claim that these comparisons are very close.  In terms of RTOS offerings that claim to support POSIX interfaces and don't require an MMU, these offerings are significantly larger (50K RAM minimum size)  or  a layered offering which increases the size significantly and decreases the performance,  It is also true that although there are claims of supporting open standards, the actual system calls differ in many cases.  They are clearly not using the standardized test suites.

Linux and "you see" Linux Comparison

When Unison and DSPnano are compared to Linux and uCLinux, there is 100% compatibility in terms of APIs for threads, inter-thread communication and synchronization, file I/O, socket I/O and stdio.  From a functional point of view, this offers a high degree of functional compatibility right to the error codes that are returned.   The main differences here are that Linux and uCLinux in their smallest configurations require many megabytes of memory.  Absolute minimum configurations without any application take 2MB of Flash and 4MB of RAM.   In comparison, the following table shows memory requirements of Unison for simple applications which include timer, clock and interrupt features.

Clearly, Unison offers an Ultra Tiny RTOS Size that is on the order of 40-400 times smaller than either Linux or uCLinx due to its modular microkernel architecture and careful space saving algorithms and designs.

Proprietary RTOS Comparison

There are a few proprietary RTOS solutions that offer similar functionality with custom APIs that are shared by no other operating systems.  These offerings are on the same order of size as Unison and DSPnano.  For some applications they may be a few bytes smaller and for other applications they are a few bytes larger.   The interesting part of this is that these systems do not offer the same level of functionality and completeness from a programming API point of view. 

Unison and DSPnano offer an ultra tiny RTOS size without compromising features or open standards.