There's always questions on the casualty figures but this one is particularly interesting given the Russian source:
In effect, it appears Russia has the lost the equivalent to 1 1/2 U.S. Army divisions in less than a month.
...and it's strongly suggested that may still be a low number.
I've read a number of viewpoints as to contributing factors behind these huge losses. In addition to some of the more obvious things, like poor equipment, training, and tactics, another recurring theme is lack of medical support and casualty evac. One source questioned whether anyone had even seen any Russian equipment with red crosses on it. Supposedly, the 40 mile convoy outside Kiev may've contained a lot of med supplies before it was hit. Losing that would further inhibit Russia's ability to treat and evac wounded.