umfangreich
extensive
Large in terms of scope, content, or extent; very comprehensive or detailed.
Die Polizei hat umfangreiche Ermittlungen in dem Fall eingeleitet.
The police have launched extensive investigations into the case.

extensive / corpulent
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extensive
Large in terms of scope, content, or extent; very comprehensive or detailed.
Die Polizei hat umfangreiche Ermittlungen in dem Fall eingeleitet.
The police have launched extensive investigations into the case.

The word is made up of 'Umfang' (scope, circumference) and 'reich' (rich). So it literally means 'rich in scope/circumference,' which can refer both to abstract things and to body circumference.
Für das Projekt ist eine umfangreiche Recherche notwendig.
Extensive research is necessary for the project.
This adjective often describes abstract nouns. Common combinations are: umfangreiches Wissen (extensive knowledge), eine umfangreiche Sammlung (an extensive collection), ein umfangreiches Angebot (an extensive range), or eine umfangreiche Analyse (an extensive analysis).
Das Museum besitzt eine umfangreiche Sammlung moderner Kunst.
The museum has an extensive collection of modern art.
The meaning 'corpulent' is rare and somewhat outdated. You can recognize it when the adjective refers directly to a person's build. In almost all other cases, the meaning 'extensive' or 'detailed' is intended.
Der Roman beschreibt einen älteren, umfangreichen Herrn mit Schnurrbart.
The novel describes an older, corpulent gentleman with a mustache.
big / great
'groß' and 'umfangreich' can both express that something takes up a lot of space, has a lot of content, or has a strong degree. 'Groß' is more general and can refer to body size, area, importance, or intensity. 'Umfangreich' is used more for things with a lot of content or many parts, for example reports, investigations, tasks, or information. A house is groß, but a report is umfangreich. When it is about length, details, or many individual points, 'umfangreich' is more precise.

Vor dem Termin liest Frau Richter einen umfangreichen Bericht.
Before the appointment, Ms. Richter reads an extensive report.
Sie braucht für ihre Familie einen großen Tisch in der Küche.
She needs a large table in the kitchen for her family.