Kim, your plant is etiolated. That is, it wants much more sunlight so it doesn't grow in an elongated fashion. Next, while it is hard to see your potting mix, it looks like a very heavy peat based material which will retain too much water and potentially rot your plant's roots. It should be mixed with 35% ~ 50% perlite. And finally, yes it is natural for bottom leaves to die off as the plant grows. I would cut the two plants in the back in half, saving the tops and allowing them to callous for several days and re-planting. With some weather, they should root in a week or two. New stems will grow from the remaining stems and with good light should grow tighter. Having said all of this, this plant's normal growing form may be just as it is; skinny.

Needs more sun!

Needs more sun!