Last week I posted a question of the Sha'ar HaMelech to which R' Unterman, in an article in the Sefer haYovel for R' Shimon Shlop, found R' Shimon's answer so compelling that he and other talmidim were completely taken by R' Shimon's derech. I'm not going to rewrite the whole kashe -- here is a link. We left off as follows: If a shevua on chatzei shiur is chal on top of the issur neveilah because it adds the potential for malkos for eating a smaller shiur than a k'zayis, why is the issur neveilah not chal on top of the issur achila of Yom Kippur -- the issur achila on Yom Kippur is on a k'koseves, but neveilah adds malkos for a smaller shiur of k'zayis?
As I predicted, the Brisker answers are first out of the gate with a gavra/cheftza chiluk. The issur shevua is chal because a shevua is an issur gavra, as opposed to neveilah which is an issur cheftza; Yom Kippur and neveilah are both categorically identical issurei cheftza. I don't want to get too bogged down in a discussion of this sevara, but there is certainly room to debate whether the issur of food on Yom Kippur, which is only for one day, can really be called an issur cheftza (see R' Yosef Engel in Esvan D'Oraysa regarding issurei zman). And the sevara begs the question: why can categorically different issurim be chal when these categories make no practical difference in terms of what the person may or may not do?
R' Shimon took a different approach. Even if two issurim have different shiurim, they cannot overlap. Since chatzi shiur asra torah, the smallest piece of neveilah is already prohibited because all food in any shiur may not be eaten on Yom Kippur. Why then is the issur of chatzi shiur chal because of shevua when chatzi shiur of neveilah is already assur? Because in that case the consumption of the food per se is not the sibas ha'issur -- the sibas ha'issur, the root cause of the law, stems from the violation of one's oath that the eating demonstrates.
You can see the difference between R' Shimon's approach and a Brisker approach. R' Shimon is less concerned with categories, and more willing to speculate on the why, or the root cause of issurim. R' Yosef Engel has a whole essay on differences between kamus and eichus and one can easily see that distinction working here almost as a middle ground between R' Shimon and Brisk: the issurim of neveilah and Yom Kippur are different in kamus, while shevua is different in eichus.
Parenthetically, in the past I've made reference before to R' Shimon's first shiur on Nedarim in the chiddushim in which he attempts to distinguish between issurei gavra and cheftza (I'm not aware of any Brisker-authored attempts to define the terms that they helped make popular). Interestingly, R' Unterman references this specific shiur in his essay and remarks that the talmidim were not at all taken by R' Shimon's approach.