From the late '70s through 2009 there was a huge drop in the number of people who walked or bicycled, and there was also a significant drop in drunk driving over that time. Those explain most of the pedestrian fatality drop.

As far as truck sizes go, yes they have steadily increased, but the bigger trucks did not have as much effect on pedestrian fatality rates in the '80s or '90s because they were a much smaller percentage of vehicles on the road. In 1980 trucks were only about 1/5 of new vehicle sales. It was up to about 1/3 in 1990. By 2000 it was a little under 50%. By 2010 it was a little over 50%. Sometime around 2017 it was 2/3 trucks, and about 4/5 by 2023. (SUVs are counted as trucks in these numbers).

Remember those are figures for new sales, so changes in the percent of trucks on the road will lag changes in the percent of trucks in new sales.

You can get an idea of whether phones or size was the more responsible factor by comparing injuries to deaths. Phones would increase deaths mostly by increasing the number of accidents. Big trucks/SUVs would increase deaths by making accidents more fatal. In the phone case injuries and fatalities should rise at about the same mount. In the big vehicle case fatalities should rise more than injuries.

Fatalities have in fact risen more than injuries (80% vs 15%), suggesting that size is the much more significant factor.