I think there's a mind-set (that's rife in movies aswell) that everything has got to be bettered, which equates to things being bigger, other wise its not going to keep or attract an audience.
Things have to be big, explosive, eye catching, the threat has to effect a mass amount of people, it has to be the worst threat possible, the stakes have never been higher, it has to push the characters to extreme, the threat has to be near impossible to stop.
If its not bigger better and the stakes are not higher then the audience wont see the point and ultimately not like it, the audience will lose interest or it won't attract people to watch it.
The mind-set naturally leads to escalation - Series one Daleks attacks earth, series two Daleks and Cybermen attack earth and rose is lost, series three the master conquers earth and plans to take over the universe with his army from the end of the universe while having the Doctor caged up, series four Daleks and Davros invade a moved earth and are going to destroy all realities and rose and friends all come back, specials the end of time itself, series five has the Tardis and the universe blown up and the Doctor placed in a prison by all of the main alien species he's come across. The stories delve deeper into the Doctor to raise the stakes, the time war and saving galifrey, Doctor Who Doctor Who Doc-Tor Who! his name, the companions being these super important people, Doctor as a child.
One of the writers of the last Star Trek movie said something like they had to think of things they could do that would give them big and spectacular images because that was expected, they had to compete with other movies that were putting spectacular things on screen, so they came up with the enterprise rising up from the sea (when you could argue this made no sense) and have Spock inside a volcano, and ships crashing and destroying earth cities, and then there's the thing that happens to kirk.
Personally I find the so called smaller episodes to be more affecting, they don't have a mass threat they are smaller in scale and more intimate, yet are often more powerful and interesting. Empty Child, The Impossible Planet, Blink, Human Nature, Midnight, Amy's choice, Hide, The Mummy on the Orient Express none of these have massive scale but they really draw you in and are much more interesting entertaining and memorable than the big series finally episodes. I remember episodes like the End of Time or Time of the Doctor because David or Matt leaves rather than for the plot or interesting ideas
