This paper describes a measurement of the jet radius dependence of the dijet momentum balance between leading back-to-back jets in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>1.72</a:mn><a:mspace width="4pt"/><a:msup><a:mrow><a:mi>nb</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup></a:mrow></a:math> of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mtext>Pb</c:mtext><c:mo>+</c:mo><c:mtext>Pb</c:mtext></c:mrow></c:math> collisions collected in 2018 and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>255</d:mn><d:mspace width="4pt"/><d:msup><d:mrow><d:mi>pb</d:mi></d:mrow><d:mrow><d:mo>−</d:mo><d:mn>1</d:mn></d:mrow></d:msup></d:mrow></d:math> of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>p</f:mi><f:mi>p</f:mi></f:mrow></f:math> collisions collected in 2017 by the ATLAS detector at the LHC. Both datasets were collected at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:msqrt><g:msub><g:mi>s</g:mi><g:mtext>NN</g:mtext></g:msub></g:msqrt><g:mo>=</g:mo><g:mn>5.02</g:mn></g:mrow></g:math> TeV. Jets are reconstructed using the anti-<h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>k</h:mi><h:mi>t</h:mi></h:msub></h:math> algorithm with jet radius parameters <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>R</i:mi><i:mo>=</i:mo><i:mn>0.2</i:mn></i:mrow></i:math>, 0.3, 0.4, 0.5, and 0.6. The dijet momentum balance distributions are constructed for leading jets with transverse momentum <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>p</j:mi><j:mtext>T</j:mtext></j:msub></j:math> from 100 to 562 GeV for <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mi>R</k:mi><k:mo>=</k:mo><k:mn>0.2</k:mn></k:mrow></k:math>, 0.3, and 0.4 jets, and from 158 to 562 GeV for <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mi>R</l:mi><l:mo>=</l:mo><l:mn>0.5</l:mn></l:mrow></l:math> and 0.6 jets. The absolutely normalized dijet momentum balance distributions are constructed to compare measurements of the dijet yields in <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mtext>Pb</m:mtext><m:mo>+</m:mo><m:mtext>Pb</m:mtext></m:mrow></m:math> collisions directly to the dijet cross sections in <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mi>p</n:mi><n:mi>p</n:mi></n:mrow></n:math> collisions. For all jet radii considered here, there is a suppression of more balanced dijets in <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mtext>Pb</o:mtext><o:mo>+</o:mo><o:mtext>Pb</o:mtext></o:mrow></o:math> collisions compared with <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:mi>p</p:mi><p:mi>p</p:mi></p:mrow></p:math> collisions, while for more imbalanced dijets there is an enhancement. There is a jet radius dependence to the dijet yields, being stronger for more imbalanced dijets than for more balanced dijets. Additionally, jet pair nuclear modification factors are measured. The subleading jet yields are found to be more suppressed than leading jet yields in dijets. A jet radius dependence of the pair nuclear modification factors is observed, with the suppression decreasing with increasing jet radius. These measurements provide new constraints on jet quenching scenarios in the quark-gluon plasma. ©2024 CERN, for the ATLAS Collaboration 2024 CERN