What am I?
I was 10 times as big in Athens as in Rome
I'm just myself in London and New York
I traditionally count in San Francisco
I will make you lean in Geneva
Lines C and D require computer knowledge
Line A needs historical knowledge
leanin line D is the verb not the adjective
biggeris not regarded to address size, height, or volume but amount
Hint 5 (big hint!!!)
The solution is imaginary
Hint 6 (big hint!!!)
For the solution you have to read the riddle literally
I changed 'higher' to 'bigger' in line A. This is more accurate and less misleading. 'Higher' was not wrong from my perspective, though.
I changed 'I'm' to 'I was' in line A as this is a historical fact and not relevant in the present.
I rephrased the comparison a bit in line A to avoid misinterpretations.
A is X times bigger as B could be interpreted as
A = B + 10 * B = 11* B, if you are really nitpicky. But here you should read
X in Athens = 10 * X in Rome (
* is the multiplication operator)