Thickness and length
The first two comparisons of the activity give results that point in opposite directions, and that is worth noticing.
Thicker means less resistance
As the cross-sectional area of the conductor increases, the resistance decreases. Wires 1, 2 and 3 show it: the thickest nichrome passed the largest current.
Longer means more resistance
As the length of the conductor increases, the resistance increases. Wires 5, 6 and 7 show it: the longer iron wire passed the smaller current.
The pipe again
If it helps, the water pipe from the start of this chapter behaves the same way. A wide pipe lets more water past than a narrow one, and a very long pipe delivers less than a short one. Resistance is what a conductor does to oppose a flow, and it responds to thickness and length just as a pipe does.
Area, not diameter
The factor is the cross-sectional area, not the width across. Double the diameter of a wire and you have roughly four times the area, so the resistance drops to about a quarter rather than a half.
Fatter and shorter carries more
Resistance falls as a conductor gets thicker and rises as it gets longer. That is why mains cables carrying a large current are thick, and why a long extension lead performs worse than a short one.