Patience, good nutrition, proper temp, humidity and quality water and did I say patience.....lots and lots of that......at 2 weeks they should be tiny-but its not uncommon for you to have fry that vary greatly in size-sometimes it can be related to stunting and sometimes it is age......eggs are not dropped at the same time-some breeders will take several hours to spawn-making them vary in age by hours and in the fish world and hour or two difference in age can be a big difference-since Bettas are born, grown and breed all within the first 3-6 months of life with an average life span of 2-3 years....
With these smaller younger fry-these guys can be at risk-mother natures way to help only the strongest survive-the larger/older fry will emit a stunting hormone that keeps them small so that they can eat them-water changes to keep this diluted and/or by separating the larger fry from the smaller fry can help to ensure they have a better chance, however, I generally leave some of mine for the larger fry-but I do pull larger fry and place in larger grow out tanks too.
I have found besides water quality and temp-nutrition being really important for good growth-I can get mine to grow out fairly fast with just newly hatched BBS fed twice a day-but I also use soil based NPT's that are full of all kinds of microorganisms for the fry to free range feed on....with a standard setup you would need to feed 3-4 times a day and with really big spawns water changes twice a day and start moving larger fry to another tank in the 3-4 maybe 5-6 week area-you don't want them to be overcrowded for too long...
