Why does the stability of YAC increase with size?
This is a question which is not easy to answer, especially the 50.000bp number (which I haven't found anywhere in there literature). However, I found some evidence, partly derived from plant and mammal artificial chromosomes (references 1 and 2), partly from the original publication from Murray and colleagues (reference 3).

The problems with small chromosomes seems to be their passage through mitosis and meiosis, where smaller artificial chromosomes tend to get lost. They are also not controlled very well in their number.

Murray states that YACs of 55kB in size are relatively stable in the yeast cells are relatively stable, but have a lower stability than natural chromosomes. When you are interested in the deeper details, I recommend reading the article. 

References:


Construction of mammalian artificial chromosomes: prospects for
defining an optimal centromere
Alteration of chromosome numbers by generation of minichromosomes –
Is there a lower limit of chromosome size for stable segregation?
Contruction of artificial chromosomes in yeast.