The problem is a matter of consistency. In ED, vessel mass generally increases at a rate well below the square of the linear dimensions, making densities fall more rapidly than expected as ships get larger, even if they are made of the same stuff.
You can make something out of whatever you like, but if you keep the thickness of the material the same as you scale the size of the object up, you still have the mass increase by at least the square of the increase in linear dimensions. In reality, thickness would also need to increase, at least to some degree, so you'd end up with somewhere closer to the ballpark of mass increasing at the same rate as volume, or the linear dimensions cubed.
For example, take a Dolphin (~51m long) and scale it to Beluga size (~203m), and I'd expect it's ~250 ton total mass to increase by somewhere between sixteen fold (to 4000 tons) at the absolute least and sixty-four fold (16000 tons) if everything scaled up proportionally. A Beluga, configured similarly to a Dolphin is only about six or seven times the Dolphin's mass despite filling about seventy times it's volume. This does not seem very plausible, irrespective of the particulars of the materials used, because it defies the cube-square law.