The aim of lightweight checkout is to only get the Jenkinsfile instead of the whole directory/branch contents.

At the moment P4Jenkins does a p4 print that is not displayed in the job log (due to known issue in Jenkins and lightweight checkout) then does a sync of the whole workspace that is visible. For example:

P4 Task: syncing files at change: 92
(p4):cmd:... p4 sync /var/lib/jenkins/workspace/LightweightPipeline/...@92
p4 sync /var/lib/jenkins/workspace/LightweightPipeline/...@92

... totalFileSize 615
... totalFileCount 2
//depot/lightweight/Jenkinsfile#1 - added as /var/lib/jenkins/workspace/LightweightPipeline/Jenkinsfile
//depot/lightweight/f1#1 - added as /var/lib/jenkins/workspace/LightweightPipeline/f1

Lightweight checkout should imply that full sync is disabled and left to the control of the Jenkinsfile