Well from the now "Legends" stories (havent read any of the new stuff yet) after the battle of Endor a decent chunk of the Imperial fleet was either destroyed (a lot of high ranking officers died on the executor) or captured (the rebels captured a few of the ISDs after the battle. But this was only a small portion of the Imperial fleet. They still had ships spread throughout the entire galaxy, from the core worlds (such as Coruscant, which at the time was still called Imperial Center, there would have been a large concentration of ships as this is the capital of the galaxy) to the Outer Rim. They still had a large force, but it was just spread out.
Also after the battle of Endor, yes this was the biggest victory for the Rebellion, they were still just a movement, they had no permanent bases, they still had relatively few ships (compared to the Empire). The Empire still had all of their capital, they still had their numerous shipyards. They still had leaders (moffs, admirals, etc) that could still command. Granted after Endor the Empire started to fragment, Moffs and admirals took the title of Warlords (or other titles with horrible names lol), and massed their own fleets.
Going by this, they can easily expand on their current technology. Also Episode VII is taking place 25 years after Episode VI, so we have no clue (yet) what happened during those 25 years. Maybe after Endor the Rebellion started gaining more momentum against the Empire, but perhaps they are now in a stalemate and both sides are pretty close in terms of military power.
That's a preconception born out of the EU. Sure we can take the remastered scenes showing celebrations across the galaxy (notably on Courscuant) but this doesn't necessarily mean the Empire is gone completely... or even that the First Order is the Empire. The First order might be a successful warlord coup of what's left of the Empire for instance. Abrams might even ignore the remastered scenes and say Coruscant is still under Imperial control.
There are plenty of reasons how the First order could be superior to the Galactic Empire. One might be that the Emperor wasn't around to spend resources on frivolous projects- like Death Stars.
The scene at the end of the special edition of Episode VI with Coruscant and the celebration was brought up in the now "legends" story (and as others have said in the "Aftermath" which I have not had the chance to read yet) was given some more info. It was talked about in one of the x-wing books, unfortunately I cannot remember exactly which one, one of the pilots of rogue squadron (either Wes Janson or Tycho Celchu, again I cannot quiet remember ) was told to have been there during the celebration. It started out peacefully, but escalated, eventually some people got hooks and ropes and pulled the statue of Palpatine down. Shortly after, Imperial troops surrounded the square and started killing those present. Coruscant was still controlled by the Empire and they were not going to let the death of the Emperor prevent them from keeping control and power.
It is nice to know that this is still part of the canon of star wars.
After reading the spoiled prelude to Aftermath, this is exactly what happens.
Also seeing how no one else has posted it you can find said preview here.