Excel, worksheet scrollbar size

I have a problem with the vertical scrollbar on one of my worksheets. Basically worksheet contains ~20000 rows and scrollbar is too big to use conveniently. However when I click and drag it, it changes to the correct size (appropriate to the number of rows).

Does anyone have an idea what can be causing this weird behavior? For the reference, workbook contains many worksheets and extensively uses VBA (so maybe it is something inside, but I checked whole project and looked at , I believe, every reference to this worksheet). Problem also occurs also in the Excel 2010 and Excel 2007 except dragging doesn't always help.

Screens: http://imgur.com/a/GPIpe

Thanks!