Regarding your first question, branched substituents can be named formally or informally. For example, the 1-methylethyl group is another name for the isopropyl. The numbers are assigned moving away from the branch point.
Regarding your second question, look at a table of axial strain energies for methyl, ethyl, isopropyl, and tert-butyl. There is a big jump between the last two. Draw them or make a model, and you can easily see why.