Why does size decrease across the sequence of human chromosomes?
You nailed it!

Nomenclature

Autosomal chromosomes

Chromosomes are numbered based on their size (in base pairs).

Sexual chromosomes

Sexual chromosomes (when they exist) are defined as the largest number regardless of their length. Note that the definition of a sex chromosome can sometimes be a little bit troublesome (see this book).

Exceptions

It sometimes happens that the first approximation of the length of a chromosome is inaccurate and that chromosomes are therefore numbered wrongly. In such case, when we realize the mistake, we just keep numbering the chromosomes the same way we did before even if it doesn't follow the standard nomenclature! This is, for example, the case of chromosomes 19 and 20 in Homo sapiens as you can see on your barplot. Chromosome 20 is actually a little bit longer (by about 4.5 Mb) than the chromosome 19. There are many other types of exceptions as well. Thanks to @Dexter  pointing out int he comments that in Drosophila melanogaster (model organism) the sex chromosome is numbered before the automosomes (autosome = non-sexual chromosome).