The Atlantis is a multi-role freighter, often it is used to resupply civilian installations, saving the need of using muliple Gas Miners and Freighters. The GTVA even use them on occasion to resupply dstroyers etc. That is why the Atlantis has those 'Gas' tanks (I won't even TRY to make a joke about 80M long dildos) on the front, and cargo pods to the rear 

 Some would say this is putting all your eggs in one basket, but statistically, it is easier to defend one large target than multiple smaller ones, and the Atlantis is not 
entirely  defenceless.
As for the Tyr, well, it was my first model, hence the simplicity of it 

 Anyway, Tyr are multi-environment ships, often taking off from planets, since even rich-kids can't get daddy to buy them a destroyer for Christmas 

 So the Tyr HAS to be aerodynamic, and with a lot of it protected from the elements. That's my excuse, and I'm stickin to it.
hehehehehe
Flipside