This is a very old issue I saw in al OOo 2.

Steps to reproduce:
1. Open attached "test.ods"
2. go to sheet 2, where you find a chart 
3. doubleclick on chart, so that black control points for the whole chart appear
4. Click on the grey chart wall area, so that green control points appear 
5. Move mouse pointer to control point at the right of the chart wall nearby the
   legend, so that the mous pointer changes to "<-->" on the control point.
5. click and drag the control point 5mm to the right
   expected: horizontal size should increase app. 5mm
   actual: as expected, that's good
6. Now click outside the chart and then again 1 times somewhere into the chart, 
   so that green control points for the whole chart appear
7. Rightclick on the chart, Context Menu 'Position and size'
8. Insert "1190mm" for Width, leave Height on 120mm
9. Scroll horizontal so that you see the right end of the chart with the legend
10. doubleclick on chart, so that black control points for the whole 
    chart appear
11. Click on the grey chart wall area, so that green control points appear 
12. Move mouse pointer to control point at the right of the chart wall 
    nearby the legend, so that the mous pointer changes to "<-->" on the
    control point.
5. click and drag the control point app 10mm to the right, so that it will
   be near to touch the legend
   expected: horizontal size should increase app. 10mm
   actual: after you loose the left mouse button the horizontal size will 
           decrease to app. 230mm
This effect will appear for horizontal width bigger than (app.) 800mm