I'm more in Olympics than in one of the German Big Three. What Am I?

I'm more in Olympics than in one of the German Big Three.

I'm bigger on the sixth rock than when engaged.

I'm variable when you call or cut a tree.

What am I?

• I advise you make up a title which is something like the current one but you chose it, 'Riddle: What am I?' is too broad but something like 'Bigger on the sixth rock: What am I?' would be perfect Commented Aug 18, 2016 at 20:22
• And welcome to puzzling! pretty good first riddle, but maybe make future ones a bit longer so they don't get closed as too broad Commented Aug 18, 2016 at 20:23

Are you a

ring

I'm more in Olympics than in one of the German Big Three.

Per celtschk's more complete explanation "The Olympic sign is five rings. The German Big Three are Audi, Mercedes and BMW; of those, Audi has four rings as sign."

I'm bigger on the sixth rock than when engaged.

Saturn rings versus a wedding ring.

I'm variable when you call or cut a tree.

When you call the phone rings, rings on a tree stump.

• Good job! Was it a bit too easy? :P Commented Aug 18, 2016 at 20:19
• @Andrew, I thought it was good for a first riddle! Nice.
– John
Commented Aug 18, 2016 at 20:21
• hehe, thanks. I'll try to think up some that take a bit more guessing! :) Commented Aug 18, 2016 at 20:25
• @Andrew Don't forget to accept the answer if it is the right one :) Commented Aug 18, 2016 at 21:24

I think you are a

ring.

I'm more in Olympics than in one of the German Big Three.

The Olympic sign is five rings. The German Big Three are Audi, Mercedes and BMW; of those, Audi has four rings as sign.

I'm bigger on the sixth rock than when engaged.

The rings of Saturn (sixth planet from the sun) are larger than engagement rings.

I'm variable when you call or cut a tree.

When called, the telephone rings; that ring is different for different phones. Trees have year rings that also differ from each other.