וגבי שביעית נמי כולי – And concerning שביעית,also etc.
The גמרא is assuming that the תנא of the משנה uses the expression 'כלל גדול', when it is followed by another (‘regular’) כלל. Tosfos will explain why in מס' שביעית the first כלל is more 'גדול', than the second כלל.
והא גבי מעשר דקתני – However by מעשר where the משנה states
The גמרא, in discussing the reason why the משנה says כלל גדול, presents the following assumption; that since this כלל of the משנה is followed by another כלל therefore the first כלל is distinguished from the second כלל by using the term כלל גדול. Both רש"י and תוספות understand the גמרא to mean that the first כלל is more encompassing than the second כלל, therefore it is appropriate to call it a כלל גדול. The גמרא first reinforces this assumption by stating that this idea is found in מס' שביעית as well. However the גמרא concludes by refuting this assumption, for in מס' מעשרות, we find in the first משנה two consecutive instances of כלל, but there is no mention of כלל גדול in the first instance.
The גמרא states that the term כלל גדול is used, only when we are discussing such הלכות in which there are both אבות and תולדות, therefore we can consider it a כלל גדול, by מעשר however there are no אבות ותולדות, therefore we do not use the term כלל גדול. תוספות will discuss several issues; a) according to some, by שביעית we are (also) not prohibited from doing the תולדות
גדול עונשו של שבת משל שביעית – The punishment of שבת is greater than the punishment of שביעית
Overview: Concerning שביעית there are two general types of דינים; a) those concerning the prohibition against working the land on שביעית, which by definition means מחובר or attached to the ground; and b) the laws pertaining to how we treat any produce that was harvested on שביעית, in which manner we may eat them etc. תוס' will be discussing that which the גמרא states that שביעית, as opposed to שבת,
ואילו פאה ליתא בתאנה וירק – And the laws of פאה, however are not applicable by figs and vegetables
מדאורייתא there is a חיוב פאה only for דגן תירוש ויצהר. The חכמים were מרבה other plants for פאה except those which אין לקיטתן כאחת ואין מתקיים, for that would cause the עניים to lose more than they would gain, since they will not be able to glean that much produce. תאנה stands out among trees, in that its fruit ripens in so many stages that it is the primary symbol of אין לקיטתן כאחת.
The reason that the גמרא excludes ספיחי סטיס וקוצה as not being 'אוכל' is to teach us that even ספיחי סטיס וקוצה that are partially edible, they are still פטור from פאה, and certainly plants that are not edible at all, are פטור from פאה.