Diverging here a little, but the bike’s frame and fork material is usually laid out in the spec sheet as well. You’re looking for, in a race bike, for an aluminium frame (sometimes called alloy) and chromoly fork. No need to worry too much about the type of aluminium used, though 6061 aluminium is common amongst entry to mid level race bikes, and 7000 series aluminium used in high end frames. In the smaller bikes you may find mild-steel as the fork material, which is perfectly fine for the little racers, but from junior up, you really want to see a chromoly fork as they are stronger and a rider on this side bike will probably be jumping stuff.
Next up, the parts to vary greatly from these two sized bikes and the handlebars and cranks. These are probably what sets these two examples apart along with the frame size. The Mini DK here has 3 inch bars (this refers to their height) and the Expert has 6.5 inches. That’s a significant difference.
Then there’s the cranks. The Mini has 145mm cranks (this refers to their length) and the Expert has 165mm cranks. A 15mm difference might not sound a lot, but it’s a massive difference in crank length and little legs need little cranks. Longer legs, longer cranks.
Other notable differences are the weight, less metal on the Mini means less weight. By just on a kilogram over the Expert and will make a difference to your 5 year old chucking it about on the track. The stem lengths are the same in these examples, though these can vary on two comparable bikes from other brands/models. So while these two bikes might look very similar in photos or images, but the spec sheets show a different story. And now we’ll try to tie all this together to match you or your rider with the right sized bike. This isn’t going to be perfect, but the knowledge base that has resulted from our combined riding and coaching experience should get you pretty close.
Remember though, this is a guide, and you'll see that there are some cross-overs in sizing on some of the bikes. This is due to variations in frame/bike sizes from manufacturer to manufacturer, and the fact that between the sizes there just isn't that great a difference in the top tube length.