knowledge - Riddle on Cities. What am I?

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

Hint 1

Lines C and D require computer knowledge

Hint 2

Line A needs historical knowledge

Hint 3

lean in line D is the verb not the adjective

Hint 4

bigger is 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

Edit 1

I changed 'higher' to 'bigger' in line A. This is more accurate and less misleading. 'Higher' was not wrong from my perspective, though.

Edit 2

I changed 'I'm' to 'I was' in line A as this is a historical fact and not relevant in the present.

Edit 3

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)

Answer

I am

I

I was 10 times as big in Athens as in Rome

In Greek numerals, iota (Greek letter I) has value 10. In Roman numerals 'I' has value 1.

I'm just myself in London and New York

In English 'I' is the personal pronoun.

I traditionally count in San Francisco

'i' is commonly used for an int counter (Silicon Valley is in the southern SF Bay Area).

I will make you lean in Geneva

is the italic element makes you lean in HTML, which was invented in CERN, Geneva.
_{(Thanks to OP for above, original was 'I' standing for italic will make Geneva (a specific typeface) lean.)}

Helpful hints:

H1) Lines C and D require computer knowledge: int counter and typeface.
H2) Line A needs historical knowledge: Greek and Roman number systems.
H3) Lean in line D is the verb not the adjective: italic type leans.
H4) bigger as in amount: $10 = 10\cdot1$

H5) The solution is imaginary: the imaginary number $i$
H6) For the solution you have to read the riddle literally: What am I?