After reading some stuff on ferrofluids, my son hypothesized that stronger magnets next to ferrofluids would create smaller spikes than weaker magnets. It turned out it was not true, although the differences were minimal and there were also several problems with taking the measurements. But whatever the right hypothesis is, what makes the difference? Here is the passage we took from somewhere on the web that is confusing to me. Before I paste it below, let me say that it raises my question 2: the relationship between magnetic field, magnetic force, and magnets. I know this is too big a question and might be answered with a simple explanation for question 1 (if there is a simple explanation...).

"The stronger the field, the smaller the spikes. In the weedy field from a ferrite magnet you'll get just a smooth mound of fluid with a few spikes where the field is strongest, but the spikes get a lot smaller when you're playing with one of the bigger neodymium magnets."