If by '10 times too large' you simply mean that the RRAs are too long, then you can use the command 'rrdtool resize rrdfile.rrd 1 shrink 100' to shrink RRA number 1 by 100 rows in the file rrdfile.rrd (take a backup first!). Note that you'll have to run this for each RRA that needs to be resized; use 'rrdtool info' to find out which RRAs are defined.
See the 'rrdtool help resize' for more details, or see the manual page.
If you want to add or remove DSs or add/remove RRAs entirely, then the only way to do it is to export to XML, modify the XML, and reimport. There is an 'rrdmerge' utility in the utils directory of v2.23beta of Routers2 that can help with other more drastic changes.