Sephadex column for alpha-amylase
The '75' of Sephadex G-75 refers to the water regain value, which is defined as the grams of water absorbed upon hydration of 1g dry powder, multiplied by 10 (see Reiland). Thus the water-regain value of Sephadex G-75 is about 7.5ml. This value refers only to the water contained 
within the gel particles and not to the water trapped outside of the particles in a packed gel (Reiland). As a rule-of-thumb, the latter figure can be estimated to 50%.

So ... 1g of dry Sephadex will give about 15 ml of packed gel

From here: 1 g swells to 12-15 mL gel  

Just a thought ...
Does alpha-amylase interact with Sephadex?  Maybe 'thinking' it resembles a substrate, and (reversibly) binding to it or otherwise interacting to cause effects in addition to gel-filtration ?