Some users complain that their data extends beyond the parameters of their browser. We have a few tips to help you with this issue:
1. Only include data columns that you actually need in the report
2. Scroll at the bottom or use the left/right arrows to scroll to get to your data
3. Change your zoom setting in the browser to a smaller value
4. Close the Salesforce.com panel to the left (you need to set up this feature in Setup)
Salesforce.com changed their design template for Lightning and made the standard fonts bigger...for reports, this is not always good.
Check out this video for more details about how to solve this problem.