数字矩阵占用的内存远远超过应有的数量--R

I am creating a document term matrix (dtm for short) for a Naive Bayes implementation (I know there is a function for this, but I have to code it myself for homework.) I wrote a function that successfully creates the dtm, the problem is that the resulting matrix is taking up too much memory. For example a 100 x 32000 matrix (of 0's and 1's) is 24MB in size! This is resulting in crashy behavior in r when trying to work with the full 10k documents. The functions follow and a toy example is in the last 3 lines. Can anyone spot why the "sparser" function in particular is returning such memory-intensive results?I am creating a document term matrix (dtm for s