Well, no-one really knows what an ammonite looks like as no soft-bodied fossils have ever been found. This is in itself somewhat odd when one thinks that ammonites are found in such locations as Solnhofen in Germany where soft-bodied fossil vampyromorphs have been found displaying the soft parts. Maybe there was something unique or especially delicate about the ammonite creature itself?
Anyway, I don't see any reason why these giant ammonites could not have had thick arms like this, after all, the living chamber would be quite massive and able to support a large bulky animal. It's hard to imagine something this massive feeding on plankton, though one can imagine it hopping along the seabed hunting crustaceans. I'm sure the smaller forms were drifters feeding on plankton with thin delicate arms as you say, Jean.
There is some evidence to suggest that the ammonite may have born a much closer resemblance to the coleoids (esp the octopus) than the nautiloids based on similarities in the radula. Personally, I suspect that there there was a great variety in size and shape of the soft-bodied parts adapted to differing lifestyles, but all based on the ten-armed plan. But no-one really knows for sure........yet.
Not the biggest ammonite in the World but the biggest ammonite in my living room (and I think its a whopper!). A Coroniceras (or Paracoroniceras) from Lyme Regis in Dorset - they do get bigger than this one. The middle few whorls are restored as they rarely preserve right to the centre I'm told.
Great ammonite Neuropteris, it is at least a double quarter pounder
As for the reconstruction, just the opposite of what I imagine they would have looked like. I don't know why, but for some reason I see ammonoids more like a long slender spider web hanging from the shell.