Invasion #'s don't make sense...

There are a bunch of modifiers, check on the screen just before the attack. The least of which is the tech bonus you can get. In addition there is the mode of attack which really can shift things.

I've killed 30k with only 1k when I way ahead in tech with just a straight tech.

Well, in the not so politically correct universe of this game your 5 billion soldiers are going up against 5.1 billion aliens _total_. That's men, women, children, the elderly, whatever it is they have, and in that situation, they all die

Assuming equal tech, it takes roughly 3X the defending population to hold back the attackers.

If the game didn't make this potentially unrealistic compromise (i.e. nothing stopping you from turning 100% of the population the transports ship out from into soldiers), invasion would be impossible. How would you like to need equal or superior numbers for *every* assault? If that was the case you'd just culture bomb or terror star everyone rather than go through that painfully slow process.

Plus I think its been stated that a transport represents an armada of ground attack craft and support units.
Also you cant invade without killing all the enemy ships in orbit so you can imagine that your ships will help out with things like orbital bombardment,recon,sensor jamming etc.

It was a single combat transport using traditional warfare - that's why the #'s made no sense, because based on the info it provided, we should have won... (I tried the scenario many times - always failed).

The senario that you described is 5 bil soldiers vs. 5.1 bil civilians. I bet on the soldiers.