There are two things that will make them seem heavier in comparison to a 45g MX..
First thing is, MX Reds, for example, are 45 at actuation...at startup they're a lot less..closer to 30g. From there it ramps up to 45g at actuation which is actually halfway down. At that point you have momentum as well..
Compare this to Topre which is "45g" to get over the bump which is all at the initial stage of pressing down they key.
The second reason is, from what I can see, most "45g" Topre require closer to 50-55g to actually get over the initial bump...
It also depends on what keyboard you're using...To me there is a difference between 45g on a RF and 45g on a HHKB...sometimes the difference feels very obvious..some days less so...