Sn2 Reaction

Although ethyl bromide and isobutyl bromide are both primary halides, ethyl bromide undergoes SN2 reaction more than 10 times faster than isobutyl bromide.  When each compound is treated with a strong base/nucleophile (CH3CH2O-), isobutyl bromide gives a greater yield of elimination product than substituted products, whereas ethyl bromide gives a greater yield of substituted product than elimination product.  Explain the factor(s) that accounts for these results.