Why would you say it’s not done by population? Quoting from Amendment CI of the Constitution of the Commonwealth of Massachusetts:

> The Senate shall consist of forty members. The General Court shall, at its first regular session after the year in which said census is taken, divide the Commonwealth into forty districts of contiguous territory, each district to contain, as nearly as may be, an equal number of inhabitants according to said census

And even a casual glance over the state senate district lines would confirm this to be the case.