Looking at historical data for the House of Reps voting
2000- R 221 seats, 47.0M votes : D 212 seats, 46.6M votes
2002- R 229 seats, 37.3M votes : D 205 seats, 34.0M votes
2004- R 232 seats, 56.0M votes : D 202 seats, 43.0M votes
2006- D 233 seats, 42.3M votes : R 202 seats, 35.9M votes
2008- D 257 seats, 65.2M votes : R 178 seats, 52.2M votes
2010- R 242 seats, 44.8M votes : D 193 seats, 39.0M votes
2012- R 234 seats, 58.2M votes : D 201 seats, 59.6M votes * Democrats got 1.4M more votes but Republicans won 33 more districts
2014- R 247 seats, 40.0M votes : D 188 seats, 35.6M votes
2016- R 241 seats, 63.2M votes : D 194 seats, 61.8M votes
The first obvious thing is that more people come out to vote in presidential election years, then there is a drop off in the off year.
While all of the House of Reps are up for election every two years.
Democrats won the House in 2006 in W Bush's second term by having nearly as much turn out as they did in 2004 (43.0 to 42.3) while the Republicans had a big drop off (56.0 to 35.9).
In Obama's first election the Dems really turned out with 65.2M votes but disappeared two years later with only 39M showing up.
The Republicans took back the House by having less of a drop off.
Dems out voted Repubs in 2012 but the 2010 census Gerrymandering gave more seats to the Rs.
So how can the Democrats win more seats in 2018?
The Republicans are not going to have 63M come out to vote for them again like last year.
If the Democrats get 50M people back to vote, they should win more seats and control the House.