If something matters, it has to matter to somebody, that is why it so readily takes a Circumstance of Angle (a grammatical reactance that brings in a second participant, if outside the nucleus):
It matters to mecf. the less likely (yes, I know, I need hard evidence....)
It's big to me.
This is because mattering is evaluative while size is (construed as) objective - and it belongs in appreciation: impact. Now, appreciation and relational clauses are a pretty congruent mix, but as we know from Appraisal, appreciation: impact is borderline with affect, which has some sort of congruency relationship (deliberately vague formulation...) with mental processes. So from this perspective, just as we analyse interesting as appreciation: impact with a nod to affect: satisfaction (if it's interesting someone must be interested), we could also analyse matter as relational with a nod to mental (because of the reactances and the systemic relations, not just notionality). In other words, matter itself is relational, but it entails a separate mental process.
 Here Bartlett confuses the mental Process agnate of a circumstance of Angle (to me) with a qualitative attributive Process (matters). To be clear, in SFL theory, a circumstance of Angle is agnate to a projecting mental or verbal clause. Halliday & Matthiessen (1999: 174-5):
This type of circumstance relates to projection rather than expansion, and specifically to the projecting not the projected component. Hence there is no agnate participant; instead, the Angle corresponds to the process of saying (grammatically, the projecting verbal clause in a ‘locution’ nexus) or the process of sensing (grammatically, the projecting mental clause of an ‘idea’ nexus). Thus according to the newspaper corresponds to the newspaper says; and to her students corresponds to her students think.
 To be clear, Angle is a circumstance, not a participant. However, the Range of the prepositional phrase serving as Angle does constitute an indirect participant in the Process — indirect because it is mediated through the minor Process of the prepositional phrase. Halliday & Matthiessen (2014: 329):
The preposition, it was suggested, acts as a kind of intermediary whereby a nominal element can be introduced as an ‘indirect’ participant in the main process.
 Trivially, all second participants are 'outside the nucleus', since the Nucleus comprises only the Process and its Medium.
 This is attempt to justify (or obfuscate) the ideational confusion identified above in , by arguing from the interpersonal system of appraisal: attitude.
 To be clear, the term 'congruent' has a precise meaning in SFL theory. It refers to two levels of symbolic abstraction, typically semantics (meaning) and lexicogrammar (wording), within the one metafunction, being in agreement, as is the case in the absence of grammatical metaphor. Here Bartlett applies it to the relation between interpersonal meaning (appraisal: attitude: affect) and ideational wording (transitivity: mental process clauses).