Why are thiolates more nucleophilic than alkoxides?
It's true that thiolate ion is more nucleophilic than alkoxide ion. You are going in a right way. The order of nucleophilities is according to polarisability of atom. As we know, the size of atoms goes on increasing down the group. 

atomic size: $\ce{S&gt;O}$

Due to increase in size of atoms, the electrons are more loosely bound to nucleus. Hence, the atoms are now easy polarise. As a result, electrons are easily polarisable down a group. 

Thus, $\ce{RS-}$ of thiolate ion is more polarisable than $\ce{RO-}$ of alkoxide ion. Therefore, thiolate ion is more nucleophilic than alkoxide ion.

The polarisability of nucleophile is more important in case of nucleophilic substitution reaction where the nucleophile electrons are polarised by the positive charge of electrophile and bond is formed during the reaction. 

As nucleophilicity is a kinetic factor. More stronger the nucleophile, greater is its rate constant. The following is relative rate constant of both nucleophile on reaction with iodomethane by using methanol as reference whose rate constant is taken as unity: 

$\ce{CH3-O-};~k  =  2\cdot 10^6$

$\ce{CH3-S-};~k  =  1\cdot 10^9$