The entire reason of them turning into vehicles is so that they will be in a cameo suitable for that planet they are on but here's how that process works

1. They are not able to choose a vehicle that is either to large are to small to fit into their previous mold, or at least have the same mass... This is why the vehicles they choose look similar are at least the same size as their cybertronian vehicles
2. The transformers will most likely pick the one that looks most like their previous for so that they can adapt easiest to it... They then scan the vehicle and mimic it... Not exact though
3. The reason I said not exact is because even though they may look identical the nano structure of everything is different from the actual... Therefore it's a copy but not the real thing... Kind of like a picture you can physically feel... It looks identical but isn't the actual thing

On cybertron vehicle forms were used to haul, transport, and carry weapons and such... Most of the time unless it was meant to be a hauler they had no inside opening that was humanoid size... It just so happens that earth vehicles look close to the design of the cybertronian vehicles... On earth the transformers have to have that space so that they will be in true camouflage with the world around them

Finnally I deny what he said about the slave thing... You see before the war there were no symbols on their arm or chest because they did not nead them... Optimus states "Two factions rose..." One for peace and one for war... That's why they have emblems... That's also why the autobots have a peacfull symbol of truth and wisdom, and the deceptions have a symbol of hatred and war... Hope this answers your question

The entire reason of them turning into vehicles is so that they will be in a cameo suitable for that planet they are on but here's how that process works:

1. They are not able to choose a vehicle that is either to large are to small to fit into their previous mold, or at least have the same mass... This is why the vehicles they choose look similar are at least the same size as their cybertronian vehicles.

2. The transformers will most likely pick the one that looks most like their previous for so that they can adapt easiest to it... They then scan the vehicle and mimic it... Not exact though.

3. The reason I said not exact is because even though they may look identical the nano structure of everything is different from the actual... Therefore it's a copy but not the real thing... Kind of like a picture you can physically feel... It looks identical but isn't the actual thing

On cybertron vehicle forms were used to haul, transport, and carry weapons and such... Most of the time unless it was meant to be a hauler they had no inside opening that was humanoid size... It just so happens that earth vehicles look close to the design of the cybertronian vehicles... On earth the transformers have to have that space so that they will be in true camouflage with the world around them.

Finally, I deny what he said about the slave thing... Before the war there were no symbols on their arm or chest because they did not need them... Optimus states "Two factions rose..." One for peace and one for war... That's why they have emblems... That's also why the autobots have a peaceful symbol of truth and wisdom, and the deceptions have a symbol of hatred and war.