The gemara discusses situations where one carries a grogeres which shrinks and less than a grogeres that swells whether one will be chayev for carrying it on shabbos. The assumption is that one would not be chayev if it is less than the shiur and only because the shiur for carrying is dependent on the person machshava, would we say that if the shiur changes together with his machshava he will still be chayev.
The Minchas Chinuch (Moseich Shabbos, Tochein) raises a very interesting question. What is the halacha with issur that began with less than a kezayis and then swelled up, or started as more than a kezayis and the shrunk? He cites a gemara in Menachos 54a that is pasekend in the Rambam (ma'achalos assuros 4,4) that a kezayis must be naturally the size of kezayis and must be maintained. A kezayis of forbidden food that shrinks to less than a kezayis, or less than a kezayis that swells to more than a kezyais, one would not be chayev malkus for eating it. The only case where one would be chayev is where it began as a kezayis and ended as a kezayis but shrunk and then swelled in between, that would still qualify as a kezayis. Based on this, we do not take into consideration any artificial swelling, rather we insist that it is a kezayis naturally, otherwise one would not be chayev.
The Rashba in our sugya (cited by minchas chinuch) asks on the gemara that the gemara implies that swelling makes a difference. Meaning, if one carried out less than a kezayis for planting and changed his mind to eat it but it had already swelled to more than a kezayis, he is chayev (the only rationale to exempt is that he changed his purpose). This implies that artificial swelling helps to consider it the size of a kezayis, which is against the gemara in Menachos? The Rashba answers that there is a difference between all issurim and shabbos. Regarding other issurim we require that there be a full shiur of the issur in its natural state and swelling doesn't help to create a shiur, but regarding the issur of carrying on shabbos (and other issurim of shabbos), it is all dependent on chashivus and swelling helps to give it chashivus.
The answer of the Rashba is really predicated on the gemara 77a where the gemara says that carrying out a small amount of wine concentrate that can be mixed will water qualifies as a shiur because for shabbos - מידי דחשיב בעינן והא נמי הא חשיב. The shiurim for shabbos are dependent on chashivus. This is clear from the fact that a מצניע is chayev even on the smallest amount as we find in the Mishna. Rashi 90b explains that when one carries out a small amount for a specific purpose - גלי דעתיה דטרח ואחשביה, he reveals that this amount is considered choshuv for him and his is therefore chayev for it. For the same reason that a subjective machshava helps to make one chayev on less than a shiur, the artificial swelling is also going to be chayev. But by other issurim where the subjective chashivus is irrelevant, the swelling is also irrelevant, and one is not chayev until it has a natural shiur of kezayis.