In any democratic country we can easily find more than 20,000 people going down in the streets asking the resignation of their government.
15,000 people demonstrating in Cairo (15 M inhabitants) will not mark a break of Mubarak’s regime.
Tunisia is a small country comparing to Egypt and the fall of Ben Ali didn’t change the regime, all people who replaced him are from the former government. If Mubarak falls it will lead to a total collapse of the regime in Egypt. He was Commander of the Air Force and Egyptian Deputy Minister of Defence in early 1970’s and his fall would be extremely surprising since he doesn’t lack the support of the army like Ben Ali. Furthermore, Mubarak is the main ally of Israel in the Middle East region, and the West will be ready to back him until the last NATO soldier.