Quick Fuel line question please

110 1985 original tank has one fuel pipe thats smaller than the other in circumference looks to be 6mm the other 8mm ish which is the return the big one or the small one? i really can't work it out my guess is its the big one thats the return from looking at it but they were cut when i bought it so i am guessing .
Help would be appreciated as i am nearly there now with my 300tdi transplant!

big pipe is feed or should be

Thank you James that's brilliant

As said.
Feed is 8mm ID and 10mm OD
Return is 6mm ID and 8mm OD.
Nylon air line is cheap and available in those sizes if you need more. Join with 8 or 10mm compression fittings.