Substantial, Drastic, Major
noun, な adjective
Something big in width is large. In fact, it's of a pretty substantial size. This word is used to describe a large magnitude or degree of change, increase, decrease, or difference in a given situation.
The readings for this word are both kun'yomi readings. You've learned both of these readings by now so you should be able to read this on your own.
The meeting’s documents were substantially altered.
The price of vegetables has gone up substantially due to the drought.
The development of flu vaccines led to a drastic decrease in the number of deaths.