Width of top major grid line changing when using expand=expansion() - axis line cut off

TL;DR: It's because of the clipping. Let's exaggerate your plot by giving it extra thick lines.

library(ggplot2)
p <- ggplot(data = mpg) + 
  geom_point(mapping = aes(x = displ, y = hwy)) +
  scale_y_continuous(limits = c(0, 50),
                     expand=expansion()) +
  theme_light() +
  theme(axis.line = element_line(color = '#000000', size=5),
        axis.ticks = element_line(color = '#000000', size=5),
        axis.ticks.length = unit(10, "pt"),
        panel.grid.major.y = element_line(color='#7f7f7f', size=5),
        panel.grid.major.x = element_blank(),
        panel.grid.minor.x = element_blank(),
        panel.grid.minor.y = element_blank(),
        panel.border = element_blank())
p

In the plot above it should be obvious that the top gridline appears thinner than the others. This is due to the panel clipping, that clips anything on the panel, including gridlines, to the panel area. You can turn it if as follows:

p + coord_cartesian(clip = "off")

That already looks better. As a bonus for the detail oriented among us, you can see that there is a weird 'bite' out of the top of the axis. This is because the tick and axis lines meet under 90 degree angles, but are not part of the same (poly) line (and hence aren't joined). To get this corner nice and flush, you can set the lineend of the axis line or ticks.

p + coord_cartesian(clip = "off") +
  theme(axis.line = element_line(colour = "#000000", size = 5,
                                  lineend = "square"))

^{Created on 2022-02-15 by the reprex package (v2.0.1)}

Note that this gives an apparent extension, making the x-axis line longer than the grid lines. You deal with this in two ways:

1. You can set this lineend on the axis ticks instead, but then they might start overlapping with the text. Hence, I recommend you adjust the text margins as well then.
2. You can set this lineend on the panel grid lines too.