Rashi understands that the Gemara's answer is that he is a שואל on the whole barrel and therefore חייב. Most of the other Rishonim (on the Gemara מ"א.) learn a different p'shat that since he wants the רביעית to stay in the barrel because it stays best there, we look at it as if he took the רביעית of wine out and put it back and therefore he was מחסר. In other words it is שליחות יד with a חסרון.
The ראב"ד has a tremendous chiddush here (הל' גזילה ג:י"ב) that if you are שולח יד on only part of the object you are only חייב on that part, (the ריטב"א is מדייק from Rashi here that he holds this way as well). The only time you are חייב for שליחות יד on the whole object is if your כוונא is to steal the whole object. The obvious question is that this is against the Gemara on מ"א: both by הניח מקלו ותרמילו where you are clearly only שולח יד on part of the animal (your כוונא is definitely not to steal the whole animal but to use it and be מחסר a little) and by ר' יעקב בר אבא. You see that if you have כוונא to steal the whole object then you are חייב even without a חסרון according to everyone, yet according to the Raavad this is exactly the case where there is a מח' whether שליחות יד צריכה חסרון or שליחות יד אינה צריכה חסרון. Additionally, if the only time you are חייב for שליחות יד on the whole object is if your כוונא is to steal the whole object, what exactly is the חידוש of שליחות יד? You are a גזלן anyway? This would seem to be why all the other Rishonim argue on the Raavad.