This is because of the lanthanoid contraction as in case of 3rd transition series, the filling of f-subshell takes place. The screening effect of f-subshell is least . Due to this the effective nuclear charge experienced by 3rd transition series is quite high.
The increase in size due to increase in shell numbers is overcame by the increase in effective nuclear charge.
Due to this the size remains similar so 2nd and 3rd transition series are similar.