“Modest” can describe a person, but here the meaning is: "(Of an amount, rate, or level) relatively moderate, limited, or small". source http://www.oxforddictionaries.com/definition/english/modest “moderate” means “average in amount, intensity, quality, or degree” So a “modest pace” is somewhat slower than a “moderate pace”. Update: But please note that it's very inexact to write "grew at a modest to moderate pace."

Modest describes a person; it's a positive character attribute (see also: modesty) Moderate describes the rate at which something is happening, i.e. not too fast/much and not too little/slow, kind of average