Of course, for the same material with the same field intensity, the strengh would be proportional to the mass. But there are so different materials and so different levels and ways of magnetization, that it would be hard to say.
I'd say that it depends much more on the magnetc hysteresis of the material, and if its magnetically hard or soft.
Different magnetic materials have vastly different characteristics, both in the 'strength' of that material, AND in the 'best' geometry for that material.
For instance, (and this is just an example, not taking the time to look up the real information), Neodymium may have a 'best' geometry of X length (north to south) and an area of X times 10. Strontium Ferrite may have a 'best' geometry of X length and an area of X times 40. Let's say I have a Ferrite magnet made of Y30 material, and it is of the 'best' geometry according to that material. Now, if I make another magnet that has the same area, but twice the length (thickness is more accurate here, because ferrite tends to be more 'pancake-like' in it's 'best' geometry), I would have twice the mass, but definitely not twice the field strength.
All magnetic materials can be measured by the plot of the hysteresis of the B (field strength) and H (coercivity). Although this curve shows the curves of different types of steel (just plucked it quicky off the net), the same differences can be seen with different types of magnetic materials.
I hope this helps some ... a larger magnet can be a stronger magnet, but it is not directly proportional to the mass. It depends on the nature of the material.
Kind regards ...
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