A lot of this depends on the old point of view...
The rebels are descirbed basically as a smaller military force than the Empire, but people thinking in "Earth terms" may mistake that to mean they're REALLY a small force, like only a handful of vessels or whatnot.
The thing is, in comparison to the Empire, even MILLIONS or possibly Billions of individuals, multiple fleets, tons of fighters, etc., could/would be considered much smaller than the Empire itself.
For instance, if you follow the X-Wing series, or other EU that are similar then the Rebels have their own shipyards, t heir own safeworlds, their own infrastructure beyond the Empire... It's not as vast, it's not as easily defended, and it's not as cost effective for the Alliance but it's still formidable enough to wage war within the outer rim territories with sporatic incursions and campaigns into the deeper sectors of the Imperial Space/Galaxy.
My personal opinion is the Alliance has multiple fleets, multiple bases (with the "Secret" Yavin and Echo bases being a primary military strategic "platform" with which to leap fast and devastating larger-scale engagements into Core Imperial areas that wreak havoc... Thus the bases need to be kept secret (Because they're also easily accessible by the Empire if they find them) and they're sort of a "front line" center.
But, like I said, it depends on your POV about things. Some people, for instance, take the fleet at Endor in ROTJ to be the ENTIRE Rebel Fleet at the time.
I personally feel the fleet at Endor was a composite fleet, possibly made up of the command ships and "best" picket ships of various fleets, and at the same time multiple (dozens even perhaps) Rebel Fleets are in operation elsewhere, as are multiple rebel bases... Some even known to the Empire (Like the Shipyards of Mon Calamari for instance), but which would require HUGE campaigns to overtake...
Thus, the Alliance's main tactic is still hit/fade and guerilla tactics... War of attrition will always favor the weaker side. And thus the Rebels deathly feared the Death Star... It's abilities against fleet attacks and its ability to travel and destroy uncivil worlds would make it the ultimate weapon to take what little (Baring in mind this is all in comparison to the Empire still) the Alliance had.
Think of it this way... Imagine the support ONE world like Earth could give to the Alliance. Millions of troops, tons of equipment like ships or armor, clothing, food, financial support... One planet could make just fighters and supply the Alliance with fighters for years... Now imagine how many planets are in support of the Alliance and can generate just as much or more for the Alliance War Effort.
The possibilities are pretty huge...
Now, take that and multiply it by some outrageous factor and you have what the EMPIRE is supplied with, and you see how the Alliance is only small in comparison to the Empire, but the Alliance is by no means a "tiny" military of ragtag soldiers... It's all the whole "galactic" thing that makes it a lot larger thing to consider.
The best sources I've seen for this, if taken with some grain of salt and your own creativity, are the West End Games sourcebooks and some of the video games. They tend to put the galactic conflicts into perspective on this huge scale, whereas the movies only show the conflict on a very focused/tiny measure in comparison.