LaTeX tiny fonts: lmex10 big operators do not scale down

When setting body text at 5-7pt with lmodern and DeclareMathSizes, \sum and \int stay at the 10pt design size because the OMX lmex shape is fixed, so they tower over the text and push lines apart. Redeclare it after loading the packages: \DeclareFontFamily{OMX}{lmex}{} and \DeclareFontShape{OMX}{lmex}{m}{n}{<-> s*[0.7] lmex10}{}. Also, in multicols a \nobreak after a heading does not stop the heading itself splitting across columns; set \interlinepenalty=10000 inside the group and end the paragraph inside that group.