First time breeding my budgies myself 

My mom has been doing it many years so I've been brought up breeding them 

My budgies were mating and laying eggs in the big group cage so I put them in a seperate cage with a nest box and they are brilliant she has laid 4 at the moment and doing everything right but the 1st egg in the box is a smaller than the other 3 but I candled it and it's fertile, has anyone had any luck with smaller ones hatching or do they die in egg? ....this wasn't her 1st egg altogether as she dropped a few in big cage before putting them in seperate 

Both birds are doing amazing jobs dad feeding moms etc and mom sitting on the eggs lovely 

(Front left egg in picture) 

Thanks in advance for advice 

Update it disappeared presuming hen ate it or it got crushed 

They will lay different size eggs here and there

Nothing to worry about