23b- Center of the world ירושלים or טבריה? a shrunken country?

תוספות asks how can we say that ירושלים הויא טבור של א"י, meaning the center, if so it should take 20 days to travel to the borders since we know that ארץ ישראל us 400 x 400 פרסה and a man walks 10 פרסה a day, and the Gemara other places says that it takes 15 days to walk to the border of Bavel and 1 day to walk to the ירדן which is the eastern border?
the מצפ"א asks on Tosfos that in מגילה ו ע"א it says that טבריה יושבת בטבור א"י, further the פנ"י asks where did Tosfos find a גמרא that ירושלים is the center? However, the מצפ"א says that the תנחומא in קדושים does say like Tosfos. I found this interesting in light of the Dateline discussion where all sides seem to agree that ירושלים is the center. (you can find in מצפ"א also a discussion if the other side of the Jordan should be in the 400 x 400 calculations.)
Another interesting point is that when Tosfos discusses the walking distance of the border to the east he brings proof from סוגיות that it is less then 20 days and only 1 day walk, why does he not simply say that the distance from ירושליםto the Jordan is much less then 20 days. To bring this point home you see what tosfos answers in ב"מ כח ע"ב where he says וי"ל דעם כרמים ויערים היה ארבע מאות פרסה אבל מישוב לא היה כל כך this is amazing, for he is saying that the walking distance to the Jordan was really 20 times more then it seems to be! What i'm getting at is that it seems that Tosfos holds the land shrunk! and even though today it is a short distance it's possible that it was much more. i once heard said this is the meaning of ארץ הצבי for the skin of the deer stretches. another illustration is the varied climates in Israel in very short distances, this -some suggest- is a proof that the land really should be much bigger.