I think - as a professional (PhD) engineer - that it would actually be quite expensive to achive total uniformity of amp sizes. Tolerance accuracy costs.
There is probably just a large tolerance (min and max sizes) on the glass amps....
This is purely from an engineering point of view...
However, I once had sustanon250 from Infar India that I had to score/scratch myself in order to open... the manufacturing process it seems didn't stretch to pre-scored amps...