Nokia Lumia 620
If you're keeping active then you really don't want your phone weighing you down. So having a light handset, not to mention one that's small and very portable is desirable.
In general smartphones seem to be getting bigger all the time, with the likes of the Sony Xperia Z Ultra approaching tablet territory with a 6.4 inch screen.
But it is still possible to get some fairly compact handsets and the Nokia Lumia 620 is one such phone.
With a screen of just 3.8 inches this thing is pretty small. Its full dimensions, if you're wondering, are 115.4 x 61.1 x 11mm, so while it's not the slimmest phone around it's quite compact overall.
At just 127g it's also pretty light and while it's a budget handset it's no slouch as a smartphone either. Its 1 GHz dual-core Snapdragon S4 processor and 512MB of RAM may not sound too impressive but they serve it well and Windows Phone 8 is far from resource hungry at the worst of times.