The octavia is based on the same MQB chassis as the golf. As the MQB architecture has been designed to be adaptable and Skoda aim to offer more for less, their models are usually stretched to give a bit more room. For example the Superb also uses the MQB chassis but it's stretched out larger than the Passat. (It has about the same rear leg room as an S class Mercedes)
When we were shopping for the new family car I compiled a spreadsheet of dimensions for a load of vehicles getting the data from the tech specs in Autotrader, Parkers on line guides and sales catalogues. Having a look at it, here are some stats that come out of it like the octavia estate has a bigger boot than a passat estate or 2012 A6 estate. Golf wheelbase is 2635 and the octavia is 2680. Passat is 2712. (all are mm). My basic test was to go into a dealer, move the driver's seat all the way back (my normal position) and check out the rear room. The Octavia was usefully more roomy in the back.
I then had a search saved on autotrader for a VRS of the right spec which came good after a couple of months and ended getting one just under 3 years old with every option other than power tailgate and sun roof. I bought it with my sensible head on thinking it's a good family car, but living with it I think it's a brilliant car. So well thought out. Obviously designed by people with families as it has great little touches. The trade off is a golf would be a little more agile and probably a little more refined, but has less space and costs more.
Compared to the 2005 A4 Avant it replaced its a much better car, with much more room and my wife is very happy with it. I have a mk5 GTI for commuting that I'll probably replace next year with either a mk7 GTI PP or a Leon Cupra. I'll have to see which one I like more!