Excel spreadsheets are somethime quite large. You have a limit in your iCloud storage that might be exceeded, and you have an internal memory limit in your device.
Settings > iCould > Storage and Backup will tell you where you stand overall on iCloud.
If you click on Manage Storage, you can examine how that storage is being consumed.
Odds are you have much more storage on your device than in iCould, so if necessary you can purchase additional iCloud storage.
Im guessing that it has to do with the limitations of Pages itself. Definitely check your device and icloud settings to rule out something that can be fixed. But if you still have lots of space on each, then the issue is likely that the spreadsheet is too complex for Pages (at least on the iPad) to handle. I've seen this on several complex work spreadsheets I've used. I think the complexity is just beyond what Pages can handle. Some of this might be due to the size of the memory available to Pages, so in that respect, an iPad 3 or 4 might work better than my Ipad 2, but that's just conjecture on my part.
The only alternative I've found is to use cloudon to work on those spreadsheets. The excel app in Skydrive might also work, but I haven't tried that on a spreadsheet that won't run in Pages. If you try either option & it works, or you find something else that does, let us know how it turns out.