I tried your method and the circles moved onto the lines, but they stayed exactly the same size. However, because I didn't know you could use the transform objects with 2D drawings, your method did make me try something else:
1. First I made a group of my circles.
2. I then moved the reference point of the group to be at the bottom of the smaller circle.
3. Then, using this new reference point, I moved the whole group and positioned it so that the bottom of the smaller circle sat on the end of the lowest line.
4. I then clicked on the Scale selected entities tool from the Transform toolbar, anchored the group at the bottom of the small circle, then chose the top of the small circle as the location point and the end of the highest line as the destination point. A copy of my group then appeared, but the entire thing had been reduced in scale so that the smaller circle fitted exactly between the lines. Which is exactly what I wanted!
I've added a picture to show what happened. The first diagram shows the group moved into position, the second shows the Scale tool being used, and the third shows the result after I've moved the original group out of the way.
Thanks everyone. Wouldn't have got there without your suggestions.
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