[Suicide Junkie]

Take things to 2-D for a moment, and consider a Flat universe (or planet)
goes straight along, and if there's no discontinuity, then its infinite in size.

Now have it curved around into a sphere.
Its finite in size, and if you're persistent enough to travel in a straight line you'll go all the way around and end up where you started.

You'll also notice that for large triangles, the angles DON'T add up to 180 degrees. Depending on the radius of the sphere and the size of the triangle, it will be somewhat greater than that.
(Take three points "almost" along the equator of your sphere - Three points form a triangle, but the internal angles are all "almost" 180 degrees!)
One point at the pole, and two others on the equator "east" and "west" will make 90+90+180 = 360 (!)

In practical situations of course, you only have a small triangle compared to the radius of the sphere, so you don't notice it very easily.