I don't think size has anything to do with it...
May I help you? Can I assist you? Interchangeable, but... we use the word "Help" more frequently, perhaps.
How may I help you?
Can you help me?
Help! Help! Police!
Give her a helping hand...
Helping others feels good...
etc.
I'm not very succinct tonight, sorry. Someone else will come along to explain it better, I'm sure. Glad to have tried to help you, though