The first link suggests the blast was equivalent to 1000-1500 tonnes of TNT. Ammonium nitrate is not as powerful as TNT (Trinitrotoluene); therefore you require more of it for the same effect.
Edit: The second link is probably misquoted and is also an estimation in terms of TNT. This is common; nuclear weapon yields are also stated in terms of TNT for example, despite not containing any.
According to [1]; 880kg (Edit: Though you meant to ask for 2750t, in which case, 880t). Not far off the estimated yield of the blast. Note that the effectiveness would be affected by contaminants and storage conditions.
Edit: The second link is probably misquoted and is also an estimation in terms of TNT. This is common; nuclear weapon yields are also stated in terms of TNT for example, despite not containing any.