From the earliest Neolithic, continental cattle become progressively smaller. The largest crash occurs about a thousand years after the beginning of the Neolithic. Then sometime around the turn of the 3rd millennium, cattle shrink to their smallest size, or rather their size as observed at death. Then at the beginning of the Early Bronze Age there is a marked increase in cattle size (and people size).
The authors rule out racial introgression with wild aurochs since that would obviously make the cattle bigger, not smaller. They don't seem to consider the reverse scenario, that is the earliest Neolithic cattle are artificially large and they also don't consider introgression from newly introduced domestic races, such as steppe cattle or longifrons, both of which could impact this chart.
The authors consider two probable hypotheses, one being dairy intensification (very plausible), the other is breeding springers and slaughtering cows thereby decreasing calf size due to calving issues.
I'm not sure I understand this second hypothesis because suggests a herd mix that is opposite of hypothesis 1. Also, the decrease in cow size would seem to more often cause calving problems, but whatever.
There's about fifty different potential causes for the decrease in cattle size from the beginning of the Neolithic, all a totally valid and together may help explain much of this. OTOH, it's hard to look at chart that parallels that of human size during a period of epic mobility.
Size Reduction in Early European Domestic Cattle Relates to Intensification of Neolithic Herding StrategiesKatie Manning, Adrian Timpson, Stephen Shennan , Enrico Crema [Link]
Published: December 2, 2015
Our analysis of over 28,000 osteometric measurements from fossil remains dating between c. 5600 and 1500 BCE reveals a substantial reduction in body mass of 33% in Neolithic central European domestic cattle. We investigate various plausible explanations for this phenotypic adaptation, dismissing climatic change as a causal factor, and further rejecting the hypothesis that it was caused by an increase in the proportion of smaller adult females in the population. Instead we find some support for the hypothesis that the size decrease was driven by a demographic shift towards smaller newborns from sub-adult breeding as a result of intensifying meat production strategies during the Neolithic.