What affects aliens getting stronger?

Aliens in ground missions seem to have more hp and better armor, and better weapons, and maybe better other skills and AI.

Does it depend on time or saucer size or my level of researches?

So, if I invest alot in science and get good ammunition fast, will I have advantage, or aliens will get stronger too?

Time affects saucer size, which in turn affects the quality of ground troops you face.

Thanks.

And what about xenonauts' stats rising? Is it based on number of missions or number of kills? What affects the distribution of bonuses amongst different stats? Can I focus on strength or accuracy for a dude, or it's all random?

Thanks.

And what about xenonauts' stats rising? Is it based on number of missions or number of kills? What affects the distribution of bonuses amongst different stats? Can I focus on strength or accuracy for a dude, or it's all random?

Medals can be gained by killing enemies and lasting a number of missions.

Stats have a max and min set in "gameconfig.xml", it determines how much of any stat a new soldier starts out with.

You can 'focus' it by making them do certain things such as making them carry more than normal, or by making them shoot more at aliens.

Bravery is the hardest and most dangerous for a soldier to train for as it requires the soldier to resist a psionic attack or panic, however gaining medals will increase their Bravery by 1 for each medal gained.

It contains to the Medal. Most bring +1 Bravery, some +2 and the big ones +3. Most of my Soldiers have the +1 and +2 Medals, some of them the best with +3 (like long Service Medal).