While I cannot say for sure and would need to research it more, I do consider that certain species stay in the larval stage longer than others. For example, the green frogs (Lithobates clamitans) and bullfrogs (Lithobates catesbeianus) have much longer gestation periods than others, sometimes lasting up to two years, and typically breed in the summer; green (Hyla cinerea) and gray treefrogs (Hyla versicolor/chrysoscelis) are also summer breeders, but their tadpoles metamorphose after several weeks. At the same time, the environment can also affect the rate of development, such as hydroperiod of a pool or temperature. Since you've mentioned European species, are you from Europe? And were these species that you noticed in the same location?

Thanks Nyssa for your information so far. I'm from the UK and these tadpoles were found in the same pond in Cambridge, England.

Thank you for confirming, Steven. That is certainly very interesting and I too am intrigued with what you've observed. The toads I've observed in the southern Appalachian portion of the United States, which are American toad (Anaxyrus americanus) and Fowler's toad (Anaxyrus fowleri), tend to metamorphose within the span of a couple of months, but this can even vary among latitudes and altitudes. And ultimately, these aspects can of course correlate with different environments (i.e. places being more north or south, colder or warmer). I've heard of some American bullfrog tadpoles taking a few months to metamorph in southern locations, and then taking almost 3 years in our northern states... And, thing is, those tadpoles are equipped for overwintering, which is why they can last in northern climates and not die; not every species of frog have larvae that are capable of that. 

Anyways, back to what you've observed! From what I've read on just some life history information, it seems that B. bufo tadpole development tends to take a little longer than R. temporaria... however, I also read in an article that B. bufo's average growth rate can slow when in the presence of a degree of competition (here's a URL to the article - http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1998.tb00117.x/abstract ). So, what could really be going on is a mixture of both biological and environmental aspects! I think studying further on what factors could really be at play would be really interesting. Perhaps investigate the pH level and water temperature, along with continuing to observe species presence... 

I hope this kind of helps - I could really keep pondering about it all day!

Best wishes,

Nyssa

http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1998.tb00117.x/abstract

Thanks for that Nyssa, looks like I have some weekend reading to do. I shall take a look at the paper in the morning and start to ponder things myself. I regularly take water and air temperatures whenever I survey my amphibians sites, perhaps it is time I started to take pH measurements as well.