Hi Elsie,
Kettlebells originated in Russia and were molded into "poods"; 1 pood = 16 kg (35.2 pounds). In the days of the old strongmen, 1 pood was the smallest kettlebell around. This is the reason why you see many kettlebells sold in terms of kilograms, which is reflective of the pood. Newer manufacturers such as GoFit, Power Systems, & (I believe) Gill Athletics chose to sell their kettlebells according to their pound equivalent. Other manufacturers list both the kg and lb value of the bell. At any rate, you likely won't see a 25lb to 30lb jump in bells because these are not full "pood" ranges. Pavel's "Enter the Kettlebell" has a nice explaination behind this history; I'm going off of memory
**Edited to say**: I shouldn't have made such a strong statement as "you likely won't see a 25lb to 30lb jump.." As kettlebells become more mainstream, there are manufacturers who will cater to making the jumps between kettlebell sizes more incremental. However, the point that I was trying to make is that such jumps don't maintain the integrity of the history of kettlebell training. Upon their introduction to the US in 2001 by Pavel, there weren't many choices for anything lighter than a 16kg until Pavel's company Dragon Door started manufacturing kettlebells and made sizes more appropriate for newer lifters who likely would struggle with the 16kg initially. It's nice to have more options these days and to see more people embracing this functional form of strength training & conditioning. As you probably know, you can typically swing more weight than you can press. Inventions like the kettlebell buddy by AoS are very helpful to make the transition between say 16kg and 20kg. Enjoy the numerous options available today and train hard! Before you know it, you'll be ready for the 16kg, 20kg, 24kg, and above!
At any rate, here is a conversion table that you might find informative (I don't think this cut & paste well from my Excel spreadsheet):
POOD --> Kilos --> Pounds
--> 2 --> 4.4
--> 4 --> 8.8
--> 6 --> 13.2
0.5 --> 8 --> 17.6
--> 12 --> 26.4
1 --> 16 --> 35.2
--> 20 --> 44
1.5 --> 24 --> 52.8
--> 28 --> 61.6
2 --> 32 --> 70.4
--> 36 --> 79.2
2.5 --> 40 --> 88
--> 44 --> 96.8
3 --> 48 --> 105.6
