All car rental companies have downsized their fleets in order to keep costs down and to meet consumer demand for more fuel-efficient vehicles. Midsize is a smorgasbord car category. You can anything from a Kia Spectra to a Chrysler Sebring, and everything in between. The Corolla has been Hertz's midsize exmaple for a few years here in Canada. Down in the US, the prevous body style Mazda 6 was a midsize, but then it grew in size and has gone either to Standard or Fullsize. It doesn't surprise me that Hertz hasn't updated yet considering how often we see pictures of previous-generation vehicles listed on the website...
I'd definitely send them an e-mail and let them know about your dissatisfaction. Maybe it'll prompt them to update their website a little sooner...