Not entirely sure whether they knew this info or not when Jurassic park was set or not, but presumably if they are lifting genetic material from frogs they should know the size of the genome.
The mean sizes for frogs vs birds genome is 4.68pg ± 0.13 (pg is picograms of genetic material for one complete copy of the genome) vs 1.38pg ± 0.01, meaning that the frog genetic code has about 3 times as much data. They may have felt the extra genetic material would make it easier for them to find DNA that corresponds more closely to the dinosaur DNA better.
Keep in mind that having more genetic data in the genome doesn't always make an organism more complex, it may just mean that the organism hasn't "cleaned" it genetic code and there is a lot of older left-over material from the process of evolution.
The sizes of the genomes are from Gregory, T.R. (2005). Animal Genome Size Database. http://www.genomesize.com
Another fun factoid is that the largest frog genome is 13.40pg, Ceratophrys ornata (8n), Ornate horned frog while the largest bird genome is 2.16pg, Struthio camelus, Ostrich. This means the largest genome from the frog is ~6 times as large as the largest bird genome.
I know its kinda complicated, but I hope it helps.
Also from http://www.genomesize.com/statistics.php?stats=amphibs#stats_top you can see that amphibians other then frogs can have a significantly larger genome then just frogs, they may have just settled on frogs because they are a more easily managed genome then the Gulf coast waterdog, for example, with a genome of 120.60pg (9 times the size of the largest frog).