Can you list some really good PC games less than 10gb?

There aren't many games after 2012 below 10GB so, you'll have to go look among the archived game titles.

Some of the games which I remember playing are:

First person shooting - FPS.

1. CS GO - this is among the games that most professional compete.
2. COD: modern warfare 1 & 2.
3. BattleField 2 (USA vs China - multiplayer gameplay).
4. BioShock Infinite.
5. Crysis 2.
6. Spec Ops: The Line.

Car racing.

1. NASCAR 15
2. Burnout paradise city (It's a very long campaign but, the graphics, music and game-play was just amazing).

RPG, strategy, action and adventure.

1. Batman Arkam city.
2. Assassins creed 1,2 - there are other titles but, that will depend on the configuration. (totally recommend all - except for AC Unity. That was a waste).
3. Tom Clancy’s: Splinter Cell - Conviction & Blacklist.
4. Devil May Cry - DmC.
5. Fallout: New Vegas.
6. Saints Row: 3 & 4.
7. Far Cry 3.

Sports

1. Fifa 2016 and below.
2. Pro evolution soccer (PES) 2016 and below.
3. Virtual tennis 4 - It's time pass.
4. NBA 2k14 and below.
5. Rocket League.

There are several other titles under the genre of "Serious Games" - that's used for remedial learning and other professional fields.

Now if you are looking for a game to develop a special skill then you need to consider Serious games. These games are most used for; defense training, education: helping adults and children with learning disabilities, scientific exploration, health care, emergency management, Infrastructure: city planning, engineering, and other fields.

The list I've provided above were among the best that I remember playing.

Just look in the archived section of: PC gamer, Gamespot and similar gaming forums.