True, the Euros use gold, silver, and bronze sheets,and it's true that each varieity is a bit smaller than the the previous.
"Gold" is a higher bloom strength, less is needed to gel a standard unit of water, it is the smallest sheet available
"Silver" is a bit weaker, therefore more is needed (larger sheet) to gel the same standard unit.
"Bronze" is the weakest bloom strength, and the largest sheet available,(also the cheapest) more of it is needed to gel the same unit of water.
For the others,
"Bloom" refers to the strenght of the gelatin, it is measured in a device called a "bloom-0-meter, invented by, you guessed it, a Mr. Bloom. A specific amount of gelatin is mesured and gelled with a specific amount of water and allowed to congeal. This is placed under a device that plunges a rod into the mixture, and a measurement is taken of how far the plunger goes into the gel.
The Euros use sheet gleatin becasue they find it very practical and easy to dispense and dissolve. The sheets are a hang-over of the glue manufacturing where glue (very similiar to gelatin) was cut into slices and dried on nets (this is why the sheets have the fishnet pattern as well). Much like gleatin, the glue was dissolved in water and heated, there is furniture dating back to Egyptian times that still is solid using animal glues, not to mention any piece of furniture made before 1920.
A certain Mr. Cooper in the USA (of price waterhouse cooper fame) started his business empire by making glue back in the 1800's and branched out into food grade gelatin. It was his company that first started the powered gelatin, which was used in "Jell-o". Jell-O itself has a very long and uh..."interesting" history