Right now g_format_size_for_display can produce such strings as

    1.1 KB
    50.1 KB
    914.1 KB

That is between 2 and 4 significant digits (or a wider range if you do not
consider an initial 1 a full significant digit).  That's too much.

Alternatively you can look at the worst-case rounding error for the displayed
result as varying between 5% and 0.05%

In order to produce more reasonable results, I propose that the format be
"%.0f" when the scaled number is >=99.5

That would also have the minor benefit of producing a string of more uniform
length leading to a less "bouncy" GUI when this function is used for, say,
data-left-to-transfer.