Bitrate lower than expected when publishing

Hi. Using WMM 2012, I'm trying to combine parts of Video 1 (video: 1920 x 1080, 17.287 mbps, 23 fps; audio: 128 kbps, 48 kHz, stereo) and Video 2 (video: 1280 x 720, 10.425 mbps, 23 fps; audio: 128 kbps, 48 kHz, stereo). Under "Save movie", I tried both Android 720p (1280x720, 10.0 mbps, 29.97 fps) and a custom setting made to match Video 2 (video: 1280 x 720, 10.425 mbps, 23 fps; audio: 128 kbps, 48 kHz, stereo). Nonetheless, the published videos I get are both about half the total size I expect them to be and roughly 6 mbps (which is nearly half the bitrate of the source videos). 

Does anyone have thoughts on why this is happening? Why are the bitrates of the published videos coming out so low when the source videos both have higher bitrates and the chosen "Save video" configurations specify higher bitrates? Is there some bitrate limitation in WMM or in general when one selects 1280 x 720? 

Thanks and happy holidays!