Is there a mutual or positive way to say "Give me an inch and I'll take a mile"?

I don't think it's an idiom per se, but something like "You gave me the push that got me rolling" might be a suitable metaphor.

He got a good return on his investment in you.

You provided a high ROI to him for his initial modest stake.

It doesn't have to limited to economic value.

Return on investment (ROI) is the benefit to an investor resulting from an investment of some resource. A high ROI means the investment gains compare favorably to investment cost. As a performance measure, ROI is used to evaluate the efficiency of an investment or to compare the efficiency of a number of different investments. [...]

— Wikipedia

Give me a leg up and you'll never regret it

leg up fits well with the first sentence of the OP's question

I have been given a little bit of something which has allowed me to now accomplish a lot.

From The Oxford English Dictionary, leg up:

b. An act of assistance or aid given in order to help a person improve his or her circumstances; a means of improvement or advancement; (also) the improvement or advancement in a person's circumstances resulting from this. .......

2013 Sunday Times (S. Afr.) 29 Sept. Amakhosi coach Stuart Baxter wants to give emerging talent a leg up.

As the OED says, the origin of the term is to help a rider onto a horse.

A person gives you a leg up, if, for example, she gives you a plum assignment at work, you prove yourself, and your career takes off. The second part of the situation the OP describes is:

The accomplishments mutually benefit both parties.

The requires an addition to leg up, for example:

Give me a leg up and you'll never regret it.