Why is the Litecoin blockchain smaller than the Bitcoin one?
Up until 2017, both BTC and LTC had 1 MB block limit. When SegWit was soft-forked into both protocols, the capacity increased to a hypothetical 4 MB blockweight limit. 

But the limit doesn't mean every block that's produced is 1 MB.  All it means is that it can not exceed 1 MB.  What determines the size of the block is the size of the transactions in the block.  

Let's say, the data size of a "one in" and "one out" transaction is about 200 bytes.  Now let's pretend, there are 100 transactions like this that are included in a single block.  That means the total size of this block would be about 2 kB.  This is significantly smaller than 1 MB.  

This is one of the reasons why LTC's blocksize is smaller than BTC's even though it blocks 4 times as fast. The actual blocks submitted and recorded were significantly smaller than 1 MB because there hasn't been as much activity on the network.  Another reason, like D Nguyen mentioned, is that LTC was started 2 years after BTC.