You can see the point of Tosphot in his argument against Rabbainu Chananel [Bava Batra page 26] Tosphot holds that when one buys three trees he owns16 cubit [amot] around them. And as for Bava Batra 81 where R.Yochanan said he owns the ground under and around them up until the length of  a plower plowing,--Tosphot holds that is if the trees extend beyond 16 cubits. [That is an average arms length. It is anyone's arm length but the average value.

You can see the point of Tosphot, not just to show what he is trying to prove about the statement of Ula, but also as for the actual law of ownership around the trees.

To see my point here is the Gemara. Ula said if one has a tree within 16 yards of the border, he can not bring the first fruit because of theft. The Gemara asks this: If one has one tree and its ground, he brings first fruits. Is that not even for a bit of ground? No. For 16 amot.

But if one has two trees he brings, but does not say the required formula [the start of Parshat KI Tavo]. Is that not for just a bit? No. For 16 amot.   

You can see from the first question of the Gemara that the Gemara itself is holding that when one buys a tree and its ground, he owns 16 cubits round it.

I am being a bit short here. The idea is this. R Chananel holds the reason for Ula is because of actual theft of the fruit. But he also holds the law is not like Ula. Tosphot holds the reason for Ula is that he does own the fruit but gettiing sustenance from the ground of his neighbor. Furthermore Tosphot does hold the law is that when one buys three trees, he gets 16 amot around them.

So while Rav Shach argues for R Chananel and shows why the Rambam and the Beit Yoseph do not hold that one gets 16 cubits are the trees, I was just showing why Tosphot hold he does get 16 cubits.