Polls with Large Voter Samples All Favor Romney; Smaller, Less Reliable Polls All Favor Obama. Why?

RealClearPolitics publishes a continuously updated average of all major national presidential polls — as of this evening, their chart looks like this:

(Note that the RCP chart is updated frequently, so that by the time you read this, it may be slightly different.)

But whenever I check the RCP average, including today, I notice something odd: The larger the polling sample size, the more the poll favors Mitt Romney.

I've copied the RCP data and pasted it in here in a format that I can re-order. First, here's how RCP organizes the chart, which they do chronologically, with the most recent poll at the top:

PollDateSampleMoERomney (R)Obama (D)Spread
RCP Average10/15 - 10/23----47.847.2Romney +0.6
Rasmussen Reports10/21 - 10/231500 LV3.05046Romney +4
ABC News/Wash Post10/20 - 10/231394 LV3.04948Romney +1
IBD/TIPP10/18 - 10/23938 LV3.54447Obama +3
Gallup10/17 - 10/232700 LV2.05047Romney +3
Monmouth/SurveyUSA/Braun10/18 - 10/211402 LV2.64845Romney +3
CBS News10/17 - 10/20790 LV4.04648Obama +2
NBC News/Wall St. Jrnl10/17 - 10/20816 LV3.44747Tie
WashTimes/JZ Analytics*10/18 - 10/20800 LV3.54750Obama +3
Politico/GWU/Battleground10/15 - 10/181000 LV3.14947Romney +2

But what if we simply re-ordered the polls not chronologically, but according to sample size, with the largest at the top? This is what it would look like:

PollDateSampleMoERomney (R)Obama (D)Spread
RCP Average10/15 - 10/23----47.847.2Romney +0.6
Gallup10/17 - 10/232700 LV2.05047Romney +3
Rasmussen Reports10/21 - 10/231500 LV3.05046Romney +4
Monmouth/SurveyUSA/Braun10/18 - 10/211402 LV2.64845Romney +3
ABC News/Wash Post10/20 - 10/231394 LV3.04948Romney +1
Politico/GWU/Battleground10/15 - 10/181000 LV3.14947Romney +2
IBD/TIPP10/18 - 10/23938 LV3.54447Obama +3
NBC News/Wall St. Jrnl10/17 - 10/20816 LV3.44747Tie
WashTimes/JZ Analytics*10/18 - 10/20800 LV3.54750Obama +3
CBS News10/17 - 10/20790 LV4.04648Obama +2

Notice the unmistakable trend?

All polls with 1000 or more respondents favor Romney; all polls with smaller than 1000 respondents favor Obama (or are tied).

Statisticians will tell you that the larger the sample size, the more reliable the poll. This fact is reflected in RCP's "Margin of Error" (MoE) column, which shows a lower margin of error, and thus a greater level of reliability, for the large-sample pro-Romney polls. Each and every pro-Obama poll has a higher margin of error, and is thus less reliable.

These are the facts as they currently stand, and they've been true like this almost every day since soon after the first debate when Romney surged in popularity.

But Why?

The question we must now ask ourselves is why only the weak polls with low response rates favor Obama.