http://upload.wikimedia.org/wikipedia/en/c/c7/Pillar_of_autumn.jpg
http://images2.wikia.nocookie.net/halo/images/b/bf/Spirit_of_Fire.JPG
Assuming they use the same sized windows, compare the two using that. The Pillar of Autumn is probably about as tall as the Spirit of Fire, but the SOF is about twice as long.
Pull them up right next to each other and you'll quickly see the differences. The Pillar of Autumn actually looks less ready for war than the SOF.
According to Halopedia, the Spirit of Fire is in fact a Colony Ship, but equipped with heavy weapons and armaments. A colony ship is built to be enormous, capable of packing lots of people and supplies in them. The primary difference between the PoA and the SoF is that they were built to be different things. Ones as small as humanly possible, making it quick and maneuverable in combat, as well as more dense and better combat ready. The other's a sprawling vessel that's built to be enormous, capable of packing entire colonies in it. The PoA would probably kick the crap out of SoF in combat, but the SoF has A LOT more troops and vehicles.
Proof:
"The UNSC Spirit of Fire (CFV-88) is a modified UNSC Phoenix-class Colony Ship built around 2418. Its Ship Emblem reads "Exitus Acta Probat," Latin for "The result justifies the deed." It was modified for wartime use against the Covenant. Spirit of Fire's onboard A.I. is named Serina.
In Halo Wars, the player will be able to command the troops of the Spirit of Fire including Warthogs, Scorpion Tanks, Marines, and at least five Spartans from SPARTAN Group Omega.
The UNSC Spirit of Fire is over 100 years old, dating back from 2418." - Halopedia
EDIT: The Master Chief has an air-tight suit. He probably wouldn't drown before he ripped the door off his HEV.
The HEV's were ridiculously dangerous to use. The ODST's were the craziest of the marines who didn't fear death. I think most mortality rates for the ODST's occurred when they hit the earth in the pods rather than in actual combat. At least that's what the books told me...