If it were the case that ring signature mixing were extensively used (it will be in the future but is not and has not been the case so far), then the chain would be around 3 times bigger than a Bitcoin-like chain with similar usage. This is derived from a combination of the mix factor (bigger signatures), denominations (more signatures), and a much tighter encoding for transactions (smaller base size).

Likewise, if Darksend were extensively used, the Dash chain would also be some similar multiple of a Bitcoin-like chain with equivalent usage (due to the additional transactions created for "rounds" of Darksend).

Thus one can conclude from the numbers you cite that some combination of these are true (probably both):

1. The difference in size is largely due to higher usage of Monero

2. DarkSend is not being extensively used.

And therefore:

3. If Dash had the same usage as Monero, and Darksend were extensively used, then the chain would be much closer in size, if not larger (possibly larger because of multiple rounds of Darksend, along with the bulkier Bitcoin-style transaction encoding)

But let's not get too far off topic here. As usual the Dashers like to pay distract-and-obfuscate when it comes to any criticism of Dash, changing the subject to talk about Monero instead. Apparently regardless of how valid the criticism or how credible the source. Monero having a larger blockchain does not make Dash any less snake oil.