Despite the Mazda 3 growing in size over the years (and gaining about 100kg depending on the model and specification) the boot size hasn’t improved in the 11 years between the models you’ve asked about.
The 2007 Mazda 3 sedan had a boot measuring 416 litres, while its 2018 opposite number could fit eight litres less with a total of 408 litres. The 2018 hatchback had even less luggage capacity with just 308 litres.
As an aside, when comparing boot sizes, it’s important to know the exact method the manufacturer has used to arrive at the figure in the brochure. There are different methods and they have a huge effect on the outcome. Presumably, Mazda hasn’t changed its measurement method over the years, so the comparison in this case stands.