Steric bulk has a major impact on SN2 reactivity.  Compare structures for calculated (PM3) transition states for the following 4 SN2 reactions.

Reactants
 TS structure
 Erel
CH3Br + -CN

SN2_TS.pdb
C-Br = 2.18
C-C = 2.02
10.7 kcal/mol
(+44.7 kJ/mol)


krel = 100,000
(experimental)
CH3CH2Br + -CN

1_SN2_TS2.pdb
 C-Br = 2.24
C-C = 2.03
+16.7 kcal/mol
(+70.0 kJ/mol)


krel = 5,000
(CH3)2CHBr + -CN
2_SN2_TS2.pdb
C-Br = 2.32
C-C = 2.05
+22.1 kcal/mol
(+92.4 kJ/mol)


krel = 1,000
(CH3)3CBr + -CN

3_SN2_TS2.pdb
C-Br = 2.44
C-C = 2.12
+30.9 kcal/mol
(+129.0 kJ/mol)


krel = 1

The results fit closely with observation:  methyl substituents react very fast (low energy barrier), followed by primary and then (slowly) secondary.  Tertiary substrates do not undergo SN2 reactions--the rate is so slow that other things happen....