While there surely are some growing glaciers, studies like these are designed to determine a global trend by ensuring glaciers from all regions of the globe are assessed. There are 67,000 glaciers in the World Glacier Inventory. Not all, or even most, have quality data for many decades, but there are enough with adequate data, located in enough regions of the globe, to know the average trend.

(source: NSIDC)

Don't forget: there is similar evidence from other parts of the cryosphere. It's also worth noting that given the right circumstances, warming can actually cause glacier growth, with accumulation of increased winter snowfall outweighing increased summer melting.

Check this page for some good before-and-after images of glaciers over the last century, as well as other images of visible effects of global warming. There are also some compelling animations of changes in Glacier Bay National Park here.