Great question!

Google states “Google Trends calculates the ratio of searches for your term coming from each city divided by total Google searches coming from the same city”. The same is said for each region. All you can really judge from these graphs is whether a search term is more popular in one area over another. These city/regional graphs represent a ratio within that given region. It answers questions like: For this region, does “dog food” represent a decent size percentage of all the possible google searches coming from this region. We can generate further answers like: yes, this search search term is popular in this region. But answering how popular comparing it to the worldwide total or trying to guesstimate an actual quantitative value is impossible with this tool. I am not aware of any keyword tool that is able to dissect volume by region.