Why is the Litecoin blockchain smaller than the Bitcoin one? I do not understand why Litecoin's blockchain size is an order of magnitude smaller than Bitcoin's. Indeed according to this link the Bitcoin blockchain requires 196.52 GB and Litecoin's 16.55 GB.

In my expectation, the Litecoin blockchain should be bigger because the block size limit is the same, but the block interval is a quarter of the block interval of bitcoin.

Is this due to the smaller user base of Litecoin compared to Bitcoin?
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.