"Rightsizing" is a euphemism. It is an evasive way of talking about "downsizing" to make it sound nicer.
"Downsizing" means "making the size of the workforce smaller," i.e. laying people off.
"Rightsizing" is usually deceptive. It means "to make the workforce the right size," i.e. the size it ought to be. This sounds nicer for two reasons. First, it carries the clear idea of an improvement. Second, it carries the idea--almost always false--that the "right" size could be any number, not necessarily smaller, but possibly larger, whatever is the "right" size.
It doesn't fool employees but perhaps it makes employers feel better about what they are doing.