unexpected out of memory allocation

[egonw]

If I start with the following SPARQL:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX prov: <http://www.w3.org/ns/prov#>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX pr: <http://www.wikidata.org/prop/reference/>
PREFIX wd: <http://www.wikidata.org/entity/>

SELECT * WHERE {
  wd:Q773449 ?propp ?statement .
  ?statement a wikibase:BestRank ;
    ?proppsv [ wikibase:quantityAmount ?value ; wikibase:quantityUnit ?units ] .
}

This returns 8 statements:

However, when I add an OPTIONAL line:

SELECT * WHERE {
  wd:Q773449 ?propp ?statement .
  ?statement a wikibase:BestRank ;
    ?proppsv [ wikibase:quantityAmount ?value ; wikibase:quantityUnit ?units ] .
  OPTIONAL { ?statement prov:wasDerivedFrom/pr:P248 ?source_ . OPTIONAL { ?source_ wdt:P356 ?doi_ . } }
}

... it tries to allocate multiple GBs of memory, which does not seem needed:

Or am I doing something silly here?