maaser tithes - Is there a practical difference between different ways of food becoming obligated in ma'aser?

The Rambam (Ma'aser 3:3), apparently based on the Yerushalmi (Ma'asrot 4:1), lists six things that make food fixed (קבע) in the obligation for ma'aser: bringing it into a courtyard, buying/selling, fire, salt, separating teruma, and Shabbat (the Ra'avad says he should have also added pickling). Later (4:2) he clarifies that all these things, except for bringing into the house, are forbidden from the rabbis.

These thing are all mentioned throughout the Mishna in Ma'asrot (3:5, 4:1-3) as making food obligated in ma'aser. However, as presented by the Mishna, there is another thing mentioned throughout (2:5, 3:3 3:8-10, 4:5) that makes food obligated, namely, taking a lot at once. For example, taking more than one fig or grain of barley at a time means that it becomes obligated in ma'aser and it can no longer be eaten informally (עראי). The Rambam also mentions this as the definition of informal eating (3:19, quoting 4:5), but not as part of the list of six things.

Is there any legal difference between the six things that make produce fixed for ma'aser, and taking many pieces of food at a time? I don't see any such distinction made by the Mishna. But in the Rambam's scheme, it seems from a superficial reading as if he makes a distinction.

For example, the Rambam says you can make someone else's produce obligated in ma'aser by doing one of the six things, but he doesn't make this explicit about taking a lot of food together at once (Ma'aser 3:7). So in this case, a literal reading would be that if you take two of your friend's figs, your friend can still eat them separately without taking ma'aser. This could be reasonable, because (as the Rambam presents it) you're allowed to eat the food informally before one of the six things happen to the food, so it might depend more on the person eating it than on what happens to the food itself. However, the Mishna itself uses for all of these cases the same wording of חייב (which, however, does have two possible meanings: "he is obligated," or "it is obligated").

So does the Rambam really mean to distinguish between taking a lot of food together, and other actions that make food fixed for ma'aser? If so, what's the Rambam's source for this distinction?