Webi and export excel and size chart

hello

My environemment is BO XI 3.1 SP6 FP6.1

excel 2010.

i have create a webi report with the bar chart.

Problem:

the chart formatting in BO is perfect but the export excel file changes things like size, frequency of labels, etc.

Have you any idea

see the example of the 2 screenshots (difference size of the chat)

Thank you