The gemara discusses the halacha when one would have a kezayis of issur or tu'mah (which the gemara assumes follows the same rules, to the exclusion of terumah and ma'aser) which shrinks to less than a kezayis and then inflates once again. The conclusion of the gemara is that if one eats issur which begins as a kezayis and then shrinks to less than a kezayis, they don't get malkus since they didn't consume a kezayis. If one eats issur which begins as less than a kezayis and grows to more than a kezayis, they also would not receive malkus. To be chayev for eating a complete shiur of a kezayis it must begin as a kezayis AND end as a kesayis. There is a machlokes when it shrinks and then swells once again to more than a kezayis, and the conclusion is that we don't apply the concept of דחוי to issurim as we do to mitzvos - therefore one who eats it after it swells back to it's original size of a kezayis would receive malkus.
The Rambam הלכות מאכלות אסורות פרק י"ד הל' ד writes:
כזית חלב או נבילה או פיגול ונותר וכיוצא בהן, שהניחו בחמה ונתמעט--האוכלו פטור; חזר והניחו בגשמים ונתפח, חייבין עליו כרת או מלקות. היה פחות מכזית בתחילה, ונתפח ועמד על כזית--אסור, ואין לוקין עליו
The Rambam seems to simply pasken like the conclusion of the gemara. However, the last statement of the Rambam seems strange - היה פחות מכזית בתחילה, ונתפח ועמד על כזית--אסור, ואין לוקין עליו
It seems that the reason it would be assur is because one has eaten a chatzi shiur. The Roghotchover (Tzafnas paneiach) asks why does the Rambam write that if the chatzi kezayis swells to be larger then a kezayis there is an issur of chatzi shiur, but no malkus since even if it doesn't swell that is the din? The simple answer to the question is that after the Rambam teaches that for swelling back to a kezayis it is restored to it's original state and is considered a kezayis, the Rambam has to write that if it never had a shiur of kezayis, then swelling would not give it a shiur of a kezayis. But, if the chiddush was simply to teach that there is no malkus then why would the rambam have to preface that it's assur - of course it is assur as chatzi shiur is always assur?