Hi there,
For myself, yes, the trend was true. But I believe it was due to my own health habits changing as I got more experienced and lived a healthier lifestyle with each pregnancy. First baby was by far the most difficult birth and he was 3,2kg, second was very easy birth and 3,6kg, third was a wonderful birth experience even though baby was in the complete wrong position for birth and his position made it a more difficult birth, but in the end when we sorted out his position, he really came through the birth path very very very quickly and he weighed 4,1 kg. Now I wore a size 32 jeans (not maternity jeans) when I go into labour, so really, size is not that important. Your body does not produce a baby that is to big to birth unless you have complications like gestational diabetes.
For a lot of my clients, their second or third babies where smaller than previous babies. One mom had 4 babies and the first one was 3,8kg, then 2,7kg, third 3,2kg and fourth 3,0kg. So no, its the general "trend" that people talk about, but its not really the truth. Each child is different and responds differently to growth hormones and thus each pregnancy and baby will follow its own pattern and has noting to do with the size of your previous baby.