Perhaps it just looked bigger?
I think the impact looked more impressive than expected in the images posted, because we have an intuitive idea of what effect such an impact would have on Earth. We're used to 1g of gravity, and when on Tempel1 you have a tiny fraction of that, the ejecta plume is massively bigger than it would be on Earth, and looks correspondingly more spectacular.
Also, perhaps the cameras were overexposing the ejecta plumes because mission scientists were expecting to see a much smaller event, and wanted to make sure they picked it up.