Travelling wave ion mobility spectrometry: determination of size?
In terms of practical data analysis standpoint, travelling wave-IMS data can generally be treated similarly as drift tube-IMS for which the technical differences is in resolution (better or worse). However, one point of consideration is that

Low gas pressure in TW IMS means strong fields in E/N terms
and thus substantial heating of ions. This may cause fragmentation
or distortion of macromolecular structures, complicating their
detection and characterization. [1]

Your first instinct to assume that the $\ce{[M + H]+}$ adduct is quite rational, however remember that ion conformation is always in flux and can be influenced by collisions with buffer gas. What you may be experiencing is the proton adduct producing a &quot;looser&quot; structure, resulting in a larger collisional cross section. The $\ce{[M + K]+}$ adduct is heavier in mass but may produce a tighter conformation.
A good example of this is in the fragmentation and mobility of polyethylene glycol (PEG) ions $\ce{[PEG + H]+}$ and $\ce{[PEG + Na]+}$. When carrying out fragmentation studies of PEG, it was found that the sodiated ion produced less extensive fragmentation and required higher collision energy.[2] Ion mobility studies have also found that PEG chelates the $\ce{Na+}$ ion similar to that of crown-ether, producing tightly wound &quot;rigid&quot; structures.[3]
These conditions are highly compound/structure dependent however, it should be something to keep in mind especially in the case of analyzing other molecules such as peptides which respond to adduction in &quot;weird and mysterious&quot; ways.
References

Alexandre A. Shvartsburg and Richard D. Smith, Anal. Chem. 2008, 80 (24), 9689–9699.
Jennifer Gidden, Thomas Wyttenbach, Anthony T. Jackson, James H. Scrivens, and Michael T. Bowers, J. Am. Chem. Soc. 2000, 122 (19), 4692–4699.
Rui Chen, Xinlei Yu, and Liang Li, J. Am. Soc. Mass Spectrom. 2002, 13 (7), 888-897.